A/C stops blowing cold
 

I have a late 1999 c1500 Silverado Z71 with 5.3 liter engine. The A/C blows cold and cycles for about 45 minutes then quits cycling and the air slowly gets warmer. I can turn it off for 5 minutes or so and it will begin to blow cold again until something heats up. I took the truck to the shop and they could not find the problem and were unable to simulate the problem. I have replaced the cycling switch on the side of the dryer and the A/C relay in the fuse box under the hood. Please advise if anyone has suggestions