1991 420 sel heater problem?
 

I have recently purchased the vehicle and found that the heater only works while car is sitting at idle,lot's of heat. When going down the road I only get cool air. I changed the thermostat thinking it may help but no change.Could this be in the switching valve i've heard so much about? Maybe a vacuum leak?

Sounds like a monovolve problem.

buy monovalve repair kit. its not expensive. i had same problem before i fixed monovalve