1985 745t

The other night I was running some errands and I noticed that my temperature guage was a little high. Normally it hits 12pm and drops to 11pm, where its stabilizes (with slight variances for heavy load, boost, hills, etc.). However, this time it was sitting up just shy of 1pm and hanging there, as though this was a new stable point.

Obviously nervous I checked everything out. I replaced the cooling system about 6 months back (everything except the pump), and I've run it through two 2400+ mile trips since, so I figured its probably in pretty good shape:

- Hoses checked out fine
- pump pulley seemed tight and pump looked fine (no leaks)
- radiator looked good
- fluid level checked out

The hose from the rad to the head wass hotter than the exit hose down below OR the radiator itself, but I figured it could just have been nothing worth noting (normally temp. conductivity to the hose, etc).

As I was driving the temperature would increase when I turned off teh car and ran an errand. At one point when I came back the temp was up around 2pm (quarter inch below red), and after I started running it STAYED at that level. However, a few minuites later it suddenly dropped back to the normall 11pm position (I think after I hit some rough road), and ran like that for awhile with forays into this same problem periodically since.

My question is what the hell is going on with my brick? I'm inclined to conclude its a sticking t-stat that isn't opening all the way or not as its supposed to anyway. The rough road and temp drop was what really led me to this conclusion (from a hypothesis) in the first place since I thought maybe the stat had just been shaken loose by the jolt.

Obviously a t-stat is an easy replace (I've got a spare), but I'd rather not deal with slightly draining the coolant to avoid massive spillage, pulling the housing, and replacing it unless it'll solve the problem. Does anyone know of something else that could be wrong? Common faults that could cause the kind of erratic behavior I'm seeing? Faulty temp. sensor? I would think a temp sensor wouldn't respond like I'm seeing, but I don't really know.

Any opinions or insight appreciated. Thanks in advance.

Best,
Rick