What I believe you have happening is one of your control doors inside of the heater/AC plemium is bleeding off.
Most of these are operated by a vacuum system and it sounds like you have a leaking vacuum line.
The bad news is there is no quick way to find it problem.
Look for the vacuum lines that control the heater-A/C functions and start tracing them out.
You may find a Hilton repair manual at your local auto parts store that will show the approximate location of these control lines
These problems often are better handled if you are already bald, cause your will be by the time you find the problem
Best of luck
SOURCE: one side air blows cold one side blows hot
GM has a known problem with the electric actuators that operate the blend and temperature doors in the HVAC system. There are 4 or 5 of these in the typical system. Any can start sticking or binding and cause this problem. Sometimes they can be cleaned, or they might need to be replaced. Sometimes resetting the HVAC system controller by disconnecting the battery for 10 minutes, then reconnecting it, will correct the problem.
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