ACC ocassionally blasts heat for about 3 minutes
 

My wifes 300D is exhibiting some strange behavior. Ocassionally when you first start it up, the ACC blasts heat for about 2 or 3 minutes, then it starts to get cold.

I discovered that the left and right hoses on the firewall (not sure what they are called.. I want to call them pinchoff hoses, since they come out of the fire wall and just pinch off and aren't actually sealed closed) were cracked and falling apart, so I replced those thinking that they might be vents to allow air out of the cab.. that were sucking heat from the engine compartment in. Could this be an actual problem with the ACC unit itself or something else?