Air Conditioning

2004 Chevrolet Silverado, same issue.
I brought my truck to a repair shop and this is what they told me:
Your vehicle has two actuators, if you have dual climate control (which is what I have). And the actuator on the driver's side is out. The driver's side is where I have my problems. It is really pricey to fix, so see if it's under a warranty or something. My estimated cost was $800.

My problems were similar to yours. My a/c, even when on full-blast cold will switch to the heater. My solution? Don't use the heater, and bundle up. Use the a/c, if it works. It's really sporadic.

(Hopefully you don't live in a place where there's seasons :) )