Starts but won't restart until cool if turned off

[hoss818]

92 GMC 4.3L TBI. Auto trans. No tilt steering wheel. 270,000 miles. Engine will start and run and drives down the road fine. Even if it only runs for 5 minutes it won't restart for about 20 minutes...although sometimes it will start right back up. Sometimes will idle fine at corner and then just shut off. (very bad at rush hour) Will start every time with a shot of ether or gas.
Fuel pressure is 14 psi. No codes generated. EGR valve working normally. Gas mileage has decreased over last year from 21+ to 18-19mpg. New ground straps installed from engine to body. When in no-start situation, engine cranks but no pulses at injectors. (it does have power to injector) If ether is used, I get pulsing while it is running(obviously). My wiring diagram shows that the pulses are done in the ECM.
My quandry is: what component tells the ECM to pulse the injectors when starting? What is working normally when cold or when the engine is running? Auto Zone says O2 sensor. I don't think so because the info I have says the O2 sensor isn't in the start sequence for a short time to allow it to heat up.
At 275,000 miles I don't want to spend hundreds of dollars randomly replacing parts to find the problem. I would like to figure out the problem though and repair it if cost efficient. Please point me in the right direction if anybody has run into the same problem and conquered it. Thanks

[MT-2500]

Welcome to AF.
You are right 02 sensor should not keep it from starting.
Any stored codes in the computer?

When no start is it getting good hot blue spark to more thanb one spark plug?

To triger injector pluse you need spark or rpm signal to pcm.

Check the ign module and dist pickup coil real close.