supercoupes

Need your opinions please:

My SC4 vibrates at rpm's between 900 and 1500 at low speeds. It is most notable in 1st and 2nd gear, it might also be present in 3rd and 4th but less obvious. I had this problem for at least 2 years now and took the car to 3 different dealerships. Two of them didn't even acknowledge the fact that there's a problem during a test drive, and the third one didn't know what to do.

Today, I turned on the AC and ..... the vibrations are gone . Further "testing" showed that the vibrations come back the moment I switch off the AC and it's super smooth again with the AC on.

Does that ring any bells? The only thing I can think of right now is that the car idles at higher rpm's when the AC is on (approx. 900-1000 rpm's vs. 700 with the AC off). But since the vibrations are normaly there up to at least 1500 rpm's, I still don't really understand what's going on.

Any ideas what to look at next? After 2 years, I'd really like my super smooth Lex back

Thanks