I noticed a new (to me) issue while driving my 1995 Impala in to work this morning. I have the temperature selector set to full hot, fan on high, function selector set to heat, vent, bi-level, etc. At idle, the air blows cold, but when the car is moving or when I bump up the RPMs while at idle the temperature comes back up. The heat is just fine while moving. I can think of two things that might change heater behavior based on RPMs: the water pump and engine vacuum. I did change out the climate control head unit a while back to fix a broken temperature selector switch; could there be a vacuum leak in the head until that produces these symptoms? The temperature guage only comes up slightly at idle and the car doesn't overheat, so I think the water pump is doing it's job. What could be going on here?