I just bought a 1990 and had this same exact issue. You won't want to hear this, but after doing the "champagne" test, I confirmed I have blown coolant seals. I did the new tstat and rad cap hoping this would make the problem go away....then had to hear the fat lady sing...

So I just bought a 1990 convertible with bad coolant seals. I have never rebuilt one of these, and can't decide if I want to or just find a used engine to throw in. Does anyone have a engine on the cheap, or anyone interested in showing me how to do a rebuild? I still have to decide which way to go....