start and run for about 15 to 20 minutes and cut out.Takes 20-60 min to restart

I would really check the Ignition Module ASAP. This type of failure is a classic symptom of a defective Ignition Module. I am NOT saying that is it for sure, but the Ignition Module switches the coil on and off to create the Spark that goes out to the cylinders. What happens is that the Power Switching Transistor gets old and when it gets hot it fails to operate until it cools down, which can take 20 minutes to an hour. So, when your Toyota dies, check for Spark, if no Spark, then inspect the Module, Ignition Coil, Distributor pick-up etc.

1991 has limited On Board Diagnostics. The engine needs three things to start, fuel (gasoline), ignition (spark) and mechanical integrity (it must develop compression in each cylinder to start ie mechanical components must be in synchronization with each other. If it had run well previous to this no start condition chances are the engine is well mechanically. Had the Check Engine light been on? The engine may have lost spark or fuel which could be due to any one of a number of sensors having failed or the fuel pump or related components are not delivering fuel to the fuel injectors. The engine uses many sensors to report engine temperature, air flow into the engine, crankshaft and crank shaft position sensors to allow the computer in your car to make calculations controlling fuel and spark to the cylinders. It is hard to speculate without being able to hear the engine crank over and do some preliminary tests to determine the cause of your no start condition. To find an intermittent problem like this I "tee into" the fuel line and tape a fuel pressure gauge to the windscreen so I can monitor fuel pressure when the problem occurs, if fuel pressure falls off you then start tracing the fuel pump, fuel pump relay and sensors that allow fuel operation. I also drive around with a timing light attached to one of the spark plug wires, when the vehicle falters I pull the trigger on the timing light and see if I have spark if not I check the ignition system and wiring and sensors associated with the ignition system. Intermittent problems are hard to find and your vehicle has limited diagnostic capabilities. Understanding how each system works, having a good wiring diagram and volt/ohm meter and patience are required to trouble shoot this kind of problem. For Toyota sensor information and theory there is brilliant Toyota factory information on http://www.autoshop101.com/ Look at left side of home page for tech articles.

All you have to do is replace coil.

My 91 did the same thing. I looked and looked for the problem. It turned out to be trash in my gas tank. I opened up the gas tank from the inside port located under the back seats. I pulled the fuel pump assembly, removed as much foreign objects, and replaced the fuel sock. Problem fixed

Check for a crack in the distributer cap. when it heats up and expands the spark could ground out. once it cools off your good for another 20 minutes :).