Great, you're at least one step ahead of me.
With the air pump and cat in place they tend to overheat -- you're better off if they've been removed early on. Be sure to note the condition of all air hoses -- it's a complex combination of carburetor and turbos (and tacked-on, 80's-style emissions equipment.)
Post when you find out what's wrong. Hopefully I can help. If you see smoke on cold start it's likely oil in the turbos. The later cars used water-cooled turbos; these are oil-cooled and run hotter so idling after a hot run is advised but not often observed.
To give you an idea, this is my 2nd biturbo. My first was my dad's that had been sitting for about a decade. After a few years (and several dollars) fighting a losing battle I gave in and bought a sorted one. It wasn't that things broke, that's expected on a 25 year old car, it's that they broke faster than I could fix them!
There's a pretty active Yahoo group:
http://autos.groups.yahoo.com/group/BiturboZentrum/
You can check there for advice and maybe find someone local to help out.