1990 5.7 350 tpi. I have seen other posts on this matter. The car runs of when cold but starts to back fire and hard to keep idle after temp goes over 100 degrees. Has anyone figured out this problem?

1) Have you verified the temp with any other method besides the in-car temp guage? These guages are well known to be innaccurate.

2) Make sure all plug wires are on securely - I had one plug wire that would not fully 'pop' on the plug, car ran 20 miles then started acting up - I had to take needle-nose pliers and slightly crimp the connector inside the plug end of the wire to make it stay on better.

3) Igntion Module - it's inside the distributor under the rotor button - it's an electronic part, and electronics don't like heat. They come new with dielectric grease - sometimes the grease will wear down, the plate will rust, causes resistance, and makes it heat up killing the circuit. Make sure to use the dielectric grease as per the instructions (and make sure it comes with the grease, sometimes falls out of box in transport).

plugs good and so is Igntion Module

Replaced throttle sensor and 02 sensor still the same.

After running for a while kill motor and try to restart and get engine lock hard to start but when is cools down start just fine. checked for vac leaks and none. will try info red temp test

What he said^

If it's hard to restart at operating temp, your gas is likely boiling.
Check timing too, I think running lean makes your engine run hotter.