83 300SD Weird Heat Problem

[cabazon]

I hav an 83 300SD . When driving the car often times there is no heat, The valve to the heater core is closed. On the climate control you can push buttons all you like with no change just lots of cold air. The only way to get instant heat is to shut off the ignition ie: all the way off and on again and Voila heat every time. Then if you push the off button anytime later. The only way to get heat again is to shut off the ignition then on again. Any suggestions here?