Intermittent Heat
 

My 2001 Boxster S, manual, 27K miles has been blowing barely tepid air, even when set on high, and then will revert to normal operation. Coolant quality and level is ok, and there are no leaks. My normally competent garage says the thermostat appears to be operating correctly. Any ideas? (And, yes, its been blowing bits of foam in my eyes ever since I bought it, but I doubt the two issues are related).

Is it affected by RPM? When was the last time the system was flushed?

Actually, they maybe related. These systems work by using doors that open and close to control both the flow of heated or unheated air movement, and these doors a sealed by foam rubber strips. When the door seals start falling apart, you can get exactly what you have described. The fix is realtively easy as the unit with the doors in it is under the dash on the passenger's side and drops straight down.

Thanks
 

That sounds plausible. I was planning to tackle the foam issue in warmer weather, but maybe I'll get an early start.

I don't see why deteriorating door seals would cause intermittent heat, maybe a constant failure. Are the HVAC doors vacuum operated? In other makes, I've seen heat fade in and out with RPMs when there's a vacuum leak.

The system controls temperature by mixing heated air (water flow to the heater core is full at all times) with cold air from the outside. When the foam seals at the edges of the flap doors start to break up, besides blowing out the vents, they jam the flaps and prevent them from moving correctly, resulting is strange heating issues.