Manual trans hydraulic clutch system

Hello, I have a 2002 chevy pick-up S-10 4cyl 5speed NV1500 trans 75000 miles. When driving the truck in hot temperatures, mostly stop and go, the clutch pedal gets stiff and is very reluctant to come back up. When you go down the road at highway speed it seems to get better.I just had installed a new clutch and pressure plate, also slave cylinder etc and new master cylinder for the clutch hyd syst. If the A/C is on it is faster to malfuction. What is wrong and can be done to fix it. Thank you Tim