How do I test to see if my truck is vapor locking? And how would I fix the issue.

When its cool or cold weather my truck will run fine. When its hot out (90-110) it will start to spit and sputter while losing power until it wont idle without feathering the gas pedal. Then after it sits and fully cools off it will start and run fine again for a while. The first time it did this I shut the truck off then it wouldnt start. It was not getting fire. So I replaced the ignition control module on the side of the distributor and the coil and it ran fine for about 2 weeks. Then started to do it again but this time the 2nd in line catalytic converter was smoking and glowing red. So I replaced both cats and muffler. It ran fine for a couple days then started to do it again. And its always the same symptoms. I tested my fuel pressure while truck was cool and it showed 40psi. My truck is supposed to have 50-60psi but runs fine on 40psi except when pulling a trailer or such up hill. Could it be a vapor lock issue? And how would I test to find out? If it is how would I fix a vapor lock issue?