Later 1999 Silverado C1500 Z71 A/C ?
 

Can someone tell me what to trouble shoot? When truck is sitting at idle with the A/C on for long periods it just started blowing warm air. If I take off the air will take a few minutes to cool again. I know this is due to not enough air traveling through the condenser, but could this also be a relay switch not sending the signal to turn the fan faster? Help

check the pressure in your ac lines

you may not have enough freon in your lines also make sure your clutch is kicking in sometimes they get stuck and make sure its not leaking (it will hiss sometimes)

also check your blend door motor (that will cause the exact problem your talking about)

A/C Blows cold and then stops
 

Thanks; I'll check those out this weekend.