1999 Engine Vibration on neutral/drive - Rough on cold start

With only 40,000 miles, our 1999 SL2 has had a rough idel (vibration) in
neutral/drive at stop, that the dealer could never fix, and its getting
worse. They've changed motor mounts, exhaust, adjusted idel speed. Still
vibrates. This last winter (Michigan) - on cold start - the engine was
very rough... until warmed. Now the rough engine is getting worse...
Thinking of bringing back to dealer again, but now that warranty is up,
I'm worried about cost when they seem to go through trial and error fixes.
Any ideas greatly appreciated.

 

Sounds like the coolant temp sensor. I bought an $8 NAPA part for my 98
SC2, but have heard the stock replacements are better made. Took me 5
minutes to replace on mine. I might add the car still vibrates like all get
out, but is better when cold!