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Q: w/ cruise on 55+ after 10 min, engine spits and sputters, no check engine light on 1998 Lexus ES300

This is an intermittent problem. Happens mostly when the cruise control is set between 55-70 mph, but has happened when held speed constant at same speed. Engine starts spitting and sputtering, gas mileage goes from 25 to 16 mpg, when flooring gas - black soot comes out the tailpipe. Car following smells heavy gas odor. Check engine light never comes on. I've replaced the spark plugs, wires, and coils, did not help. No check engine light and no codes. I drove approximately 6 hours no cruise control, did fine. Next morning set cruise control on 60 mph within 10 minutes started spitting and sputtering. Drover for 60 minutes with the problem, car sat for 6 hours (off) and started and drove 4 hours with no problem (no cruise). Oh yeah, no check engine light. When not spitting and sputtering engine run very strong with consistent 25 mpg , does not burn any oil, has 160,000+ miles . I do not understand why the sensors in the exhaust don’t pick up the unburnt gas or soot. Not sure which cylinder or whether the computer or injector is being held open??? Seems to do it more during cold weather 25-45 degrees.
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Need to Inspect the Cruise Control System. There is a way to read the data stream, so you can see in real time what the Cruise is doing, but clearly if the car runs fine with out the Cruise, well, you get the idea. Cruise problem may not set the Check Engine Light.
I was thinking it was cruise control related but it did it the other day just holding the speed constant at 65 mph. I was told that the cuise is hooked to the computer? My concern is that (of course the spitting..) but the computer didn't pick up the miss or held open injector and didn't p/u the unburnt gas in the exhaust??!! So baffled and frustrated. Love the car...
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