My 98 J30 overheated and now won't start it's got spark it wines and won't fite

SOLUTION FOUND to the following today (8/2/2010). . . just wanted to share the answer after many, many headaches, diagnostics and repairs to help others. The answer is . . . a REFERENCE SENSOR! I recently obtained a 1993 J30 which is in pretty good condition. The only big problem is with the erratic way the car runs. After the battery was charged and I drove it for the first time for about 20 minutes (it was a gift that had not been driven in a year), it ran like a dream. Acceleration was quick, the engine was smooth and handling was great. I brought it back and parked it for about 10 minutes and when I went to start it again, it would not start. The engine would turn over and all the electronics would come on, but it would not start. I need help from an expert!!! Are you up to the challenge? Here is what I've done: It had an oil change and a transmission service. It has also been given a diagnostic at the Nissan dealership. They couldn't figure out what was wrong, but suggested replacing the ECM and the Power Transducer, as well as the Coolant Sensor. Obviously, I don't want to buy these parts from Nissan on a whim, because they would be about $1500 brand new. We tried to start it under the assumption it was flooded, but it was a no go. It seemed more like the engine wasn't getting any fuel or maybe it wasn't firing completely. The weirder thing is the car backfiring and sputtering AFTER it is warmed up. Another assumption we had was about the ignition switch. It seems to be really loose. There seems to be a lot of wiggle room once the car is on and it is really tight from the off to accessory position. Could bad contacts in the ignition be causing problems in the engine? Well, warming the car up is definitely not the problem in this case. I ran over to Pull-A-Part, but the J30 there had already been stripped clean. I decided to crank the car and just let her run until the problem developed. Sure enough, the engine started vibrating heavily and sputtering about 10 minutes after the temperature gauge reached around half-way. The whole process took about 25 minutes. The dealership recommended replacing the ECU and the Power Transistor Unit. Both have been replaced independent of each other in the past, but not at the same time. Still when the units were replaced, it didn't help the problem. I've read a couple things pointing to the crankshaft or camshaft position sensor. Could the engine heating up cause this sensor to malfunction and if it did would it cause a problem like the one I described? I pulled the codes from the ECU, but all it gave me was 55. That's the code for everything is working. I pulled out the Power Transistor Unit and checked it for continuity. It was good. I attempted to pull out the Crankshaft Position Sensor, but it was difficult to get to. I grabbed some of that electronics freezing spray that you use to diagnose parts that you think are overheating. I started up the car and waited until it started acting up (20 minutes like clockwork) and then sprayed every electrical part I could think of, but nothing helped. When the problem first starts, the car will idle fine, but you can't get the engine to rev much higher. You can press the accelerator clean against the floor and it'll just cough and sputter at around 1500 rpm. It'll keep running until I shut it off, but once I do, I more or less have to wait until the next day to start her up again. I talked to somebody last night who suggested that it might be the catalytic converter becoming clogged up and causing exhaust pressure to build up. Could that be the cause? I replaced all the spark plugs using the OEM plugs. I noticed that whoever replaced the plugs before didn't use the OEM plugs and there is no telling how long ago they were replaced. I fired up the engine and waited, revving it a few times after it was warmed up, and the problem started again 20 minutes into running. This time I was actually able to rev the engine smoothly up past 1500 once the problem started, but I was only able to do it once. I shut the engine off and this time I was able to crank it again. The problem was still present after the engine restarted, but this is the first time I've been able to restart afterwards. The MAF sensor has already been cleaned, so I think it is good. The TPS sounds like it could be a candidate. I have a print copy of the service manual. I checked TPS before and after the engine had heated up and the readings remained constant. The engine once again had the same problem. Another thing I noticed this time around is there is a grinding sound when you try to rev the engine while the problem is occurring. At idle, the engine sounds fine. Then I step on the accelerator and the engine begins to vibrate and jump and there is occasionally a sharp grinding. I'm not sure what that could be. I also checked the temperature difference between the in and out pipes of the catalytic converter. I was just measuring the external temperature of the pipes with an infrared thermometer, because that's all I really had at my disposal. Once the engine was warmed up and the problem started occurring, there was about a 100 F difference between the in and out pipes. They weird thing though is that the in pipe was hotter than the out pipe. Aren't the exhaust gases supposed to heat up in a catalytic converter, so it can burn away the bad stuff? Both of the catalytic converters had the same temperature difference. They had more or less the same temperature readings too. Both were about 215 F on the in pipe and about 115 F on the out pipes. I swapped out the ECU for the old one. The car still wouldn't start. The engine will turn over and the car will backfire occasionally. There will also be an occasional grinding sound as the car tries to start. It only lasts for a second and happens right when the engine sounds like it's going to come on. I'm not sure what the sound was and I don't know if it's related to the main problem I've been having, but I do know that I can't test if the ECU is the problem if I can't get the engine started. Any ideas? I'm wondering now if it is maybe a fuel pump/fuel system problem. I want to check the fuel pressure, but I can't find where to hook up the gauge on the fuel rail.