'99 suburban heating problem

I have a wierd problem. My Sub has the rear heat option. It blows warm/hot (in the rear) according to the temp setting. The front heat coming from under the dash stays cool. It will warm up on it's own, then cool back down. While traveling on the highway, if I accelerated, I would get heat almost immediately, blow warm for a few minutes, then cool off. It happened EVERY time I accelerated. My coolant is at proper levels. Is there a "door" in that aasembly between the blower & the vents that is, maybe .....stuck? Control panel problems? What? HELP!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Thanx in advance.

It could be a couple of things. You could have a restriction in the heater core, or a bad heater water valve or the engine may not be running hot enough to sustain a hot airflow through the heater.

The heater control valve is vacuum actuated (spring loaded closed/vacuum open), if there is a leak in the vacuum line(s) or a defective valve the flow of coolant will stop when the engine vacuum is reduced like during acceleration.

This is a very common problem and not too costly to have diagnosed and repaired.

THANX EVERYONE FOR YOUR RESPONSES.....GONNA LOOK INTO THAT HEATER CONTROL VALVE.......I ASSUME IT'S THAT "UNIT" THATS SITTING ON THE HEATER HOSES PRIOR TO THEM ENTERING THE FIREWALL

Bingo!