I'm experiencing problems with my defroster/entire climate control system. I own an 89 GL wagon with AC. The problem occurs frequently when the outside temperature dips into the chilly range (40 degrees farenheight or lower). When I rev the engine to 2500 or above, the heater will stop blowing air from the defroster and switch to the panel vents. This also occurs if the system is set to "heat" or "bilevel". When engine rpm drops below the 2500 range the system will return to its previously set state. This has been driving me crazy!!! I have a hunch the problem is related to the vacuum system. I would greatly appreciate any help!! Thanks!!!