Vacuum leak?

03 Impala, 3.8L engine. Car does not blow good heat until temp gague is near normal temp, then the heat cools down noticeably when the engine is at idle (700rpm). Gets real hot when rpm is above 1500. sometimes the selection between discharge modes (e.g. defrost, floor, vent etc. can take 30sec-1min to switch over) Is this related and possibly caused by a vacuum leak? If not, what should be checked? Thanks!

Possibly. A shop that specializes in heating and AC work would be a good starting place.

The temp doors are moved by electronic actuators, the blend is done by way of vacuum. Look for the vac line to be rotten near the battery where it goes down to the vac reservoir. The temp problem could be a partially plugged heater core. Could just be low on coolant. Could be fixed by flushing with a garden hose.

Yea I would suspect a vacuum problem. However I would also be thinking possible water pump issue as well as a possible hose problem, maybe an internal collapse.