I have searched alot but didn't find anybody with quite the same problem. . .
My works fine on whatever settting I choose for about 5 minutes, after that it gets really random. It will change from hot to cold and from defrost to floor to dash vents. It is all over the place, can for example jump from defrost heat to vent AC to defrost cold to floor AC to floor heat etc. It stays on each setting from about 5 seconds to a few minutes. The fan speed also varies randomly, but I recently found that if I step on the gas the fan speed also accelerates momentarily. Any Ideas where I should start looking?
I have a 99 with auto heat and auto 4x4 (which works fine). I couldn't find any damaged or disconnected vacuum lines under the hood either.

 

With the variety of the changes that you have noted, the problem most likely is caused by a problem with the HVAC control module (also known as the ECC module - Electronic Climate Control). A high feature scan tool can be used to scan the HVAC system for codes as well as run tests on the system, but not everyone has access to one.

The vacuum control assembly only controls the mode (where the air comes out), not the air temp which is controlled by the blend door actuator. Your symptoms involving both of them seem to rule them out unless there are two separate failures involved. The vacuum control assembly is a fairly simple system of valves that shouldn't get screwy if it were to go bad, it typically just doesn't change to that particular mode.

You could try reinitializing the ECC by removing the RDO BATT fuse for 10 seconds with the ignition OFF. Reinstall the fuse and then turn the ignition ON for at least 40 seconds without starting the truck. This should allow sufficient time for the ECC to run through its initialization.

See if that helps and let us know. If it doesn't, you could try a used ECC to save some money.

 

Thanks Swartlkk
First I tried the fuse trick but it didn't work, so I switched to a new ECC. It seemed to work right on Sunday when I changed it, but it hasn't been driven more than 10 minutes at a time since. Next week I will get a chance to test it out more.