00 DHS rough idle

I just bought an 00 DHS and notice that the idle is rough. This can happen when in park or waiting at a light. Everything seems to be fine while under power. I took it back to the place I bought it from and there were no error codes. Under further inspection they found a broken motor mount and replaced it. I still have the problem. They dealership thinks it could be bad gas or water in the tank and suggested putting in some Heat. I ran the tank down, put in the Heat and filled up with a 10% Ethenol blended fuel. I have run about 3/4 of the tank through it and, you guessed it, I still have a rough idle.
Any suggestions or ideas?
The dealership is 350 miles from my home and I leave tomorrow so I would like to have this figured out before I leave. I am keeping the car regardless. Just a matter of who has to pay to get it fixed.
Thanks for any help.
ozzyn

have u done a tune up? new spark plugs? if not that you may have a blown head gasket.

I haven't had a chance to due any sort of tune up. I literally purchased this vehicle three days ago. I was thinking possible plugs as well but don't have the tools with me right now to do the change.
I hope it has nothing to do with the head gasket. Buke dollars to fix. No white smoke so hoping that isn't the deal.

How many miles are on the DHS?
I don't think it would be a headgasket. If it was, you'd notice the coolant temp higher than normal, and/or a coolant consumption.

The car has 58,300 mi. on it. I have not notices any coolant leak. Temp is dead center between C and H. The idle is not what I would call super rough. Idles fine for a few seconds then a studder. Fine for a few more seconds and a studder. Sometimes it is a 2-3 seconds before the studder and sometimes it is 3-5 seconds. Not really consistant.

I drove about 500 highway miles with her yesterday and it doesn't seem to be quite as bad now, but still there. You guys think it is tune up time? I sure it wouldn't hurt regardless.

yeah i would say tune up time.... get u some of that mobil 1 oil...bosch platinum spark plugs...thats what i get for my 96 caddy....

Enjoy your caddy!!

Will do. thanks for all of the help