Hmmm, you seem to have ruled out the t-stat, however there is a chance that they gave you the wrong one(temp wise) or that the one you got was bad before you put it in...so it might a cheap re-check to change it again. The only other thing I can think is that your water pump is failing internally and the coolant is not circulating. As far as the coolant leak...just about every engine that GM built has had intake and water pump issues. I've owned two and so far its par for the course. One had a bad intake gasket and H20 pump, and the one I have now has a seeping intake also. You didn't mention the engine size but I know the 3.1 & 3.4's are especially good for that. Just a couple things to look for. Let us know what turns up. M.
