Thanks for the reply. I had a new t-stat laying around so I went ahead and replaced it. That seems to have cured it...but it's only 75F out tonight.
Before I changed it, I noticed that with the engine cold, the upper radiator hose was still full. Am I correct that it should be pretty empty when the engine is cold?
To replace it, I just opened the housing, replaced the t-stat, buttoned it up and refilled the tank. I hope that's OK.
91 300E 120K
90 300SE 275K (sold)
92 BMW 525iM 120K
90 BMW 525iA 175K
85 300D 175K (sold)
84 300SD 245K (sold)