Runs great for 20 minutes then starts to run rough/misses occasionally

I'd start by looking for vacuum leaks, snug up the intake manifold, and take the TBI off and check the base gasket. Clean the TBI up good when it is off, get under the throttle plates and mounting surface. Use some spray carburetor cleaner on it.

Throttle Body Injection where it has 2 fuel injectors over the throat of the throttle body. It takes the place of a carburetor, sits in the same spot.

You have a TBI on there for fuel delivery.

Thank you for the photo. All the vacuum hoses are in good shape, not dried or cracked and the gasket under the TBI looks good. It occasionally will turn over but not start. Then a day later it will. One time I tried it turned over but did not start, even though the plugs were getting spark. It just started and ran fine. I got a 35 error code = IAC problem or idle error. What is the IAC problem?

Idle Air Control Valve. So that is in the picture on the left side that thing that is sticking out of the side of the TBI. That is a major vacuum port, so when replacing that you should get it loose first, then take off the TBI and clean that port out real good. When putting on the new IAC you would want to use some sort of assembly lube, like sill glide or even Vaseline around the gasket so it slides in easier. Make sure that base gasket is good.

I think this is the problem. I looked at another site that described symptoms which match what I have had. Stall while idling and it may render it unable to drive. Thank you for your help Rowefast.

Yes that could. Just get that port cleaned good. Make sure the wires are good also at the connector.

Well I tried replacing the AIC. No help. I did change the ECM got no error codes but after driving same symptoms were present. Returned the ECM and tried replacing the ICM under the distributor cap. Get no error codes but would not start with the original ECM. I am getting spark and gas so I am now have no idea what to do. Thought about getting the new ECM back but not so sure that is going to help. When pressing the brake pedal I hear a relay clicking and found the clicking relay. Any ideas.

So you are getting a nice white spark at the end of a spark plug wire?

Yes, Before all of this I have replaced my wires, plugs and dist. cap.

So you have a good spark at the spark plugs. So what is left to cause this would be a fuel issue. May want to check the fuel pressure. It should be 9 to 13 lbs. The fuel pump or the fuel regulator may be weak. You'd want to make sure the fuel filter was changed before testing.

I am in the process of getting and installing a new fuel filter. After installed, how do I check the fuel pressure?

Ok, you can rent a fuel pressure gauge from the parts store. It is important you ask them if it has all the right fittings for checking a TBI set-up. The fuel pressure gauge gets hooked up in the fuel line up at the TBI. The connections must be tight so it does not leak. You can start the vehicle and read the fuel pressure gauge, should be 9 to 13 lbs. Rev the engine up to see if the fuel pressure holds in the normal range. To test the fuel regulator, this the tricky part, you have to either block or pinch off the return line and see what the gauge is reading. The fuel pressure should go up.

Thinking about this more, that return line is a steel line. I haven't looked on my truck to see if there is a rubber section in it going back to the gas tank so blocking the return line may not be feasible. The tell tail story will be when you rev. the engine up to see if it holds the proper fuel pressure. Hold it at a higher rpm for a bit.