Mine did the same thing on my 85 300DT, I had plenty of refridge
in the system, and I was bafled, I checked the safety valve switch
on the dryer, all ok, then I found a A/C control unit, it looks like
a big relay, I opened it up and found the solder joints had failed
tried to fix, but no good. Bought a new one, it works great!
On my car it is located right infront of my fuse box, mounted on the fender.

Hope this helps!