I have a 1990 Chevy 1/2 ton with a 6.2l diesel. The a/c blows cold and especially cold when the outside temp is'nt too bad. However, if it is really hot outside or if I run the a/c for more than maybe 30 minutes, it suddenly stops blowing cold and only blows semi-cool, not nearly cool enough to keep from sweating in this hot Texas spring weather. Then, if i shut off the truck for about 30 minutes, and restart it, the a/c blows freezing cold again like nothing is wrong. Any ideas as to what could be the problem?

 

If it is over charged it could be freezing up the evaporator core.

Make sure the evap drain in the heater box to the outside is clear and working.

Does airflow get reduced as well?

Also make sure that the cooling stack(rad,condenser,oil cooler and trans cooler)is totally clean,they tend to collect a lot of debris between each pannle of the stack and limit airflow,if the condenser cant get enough air it cant remove the heat collected by the R134A.

 

Yes the airflow also gets reduced. I will check the other things you mentioned this week.