I've got a 3.0 V6 automatic XLT. About a month ago, I noticed that the tachometer was bouncing between 500 and 1000 rpms. The check engine light did not come on. I took it to a mechanic buddy who borrowed a scanner and we got no codes. We did notice the radiator was ice cold after idling for 20 min.

We'll fast forward to a week ago, after a radiator replacement and a NC state safety inspection. Sometimes out of a stop when I apply the gas the car shakes, then resumes smooth acceleration. Today while driving around the city it shakes out of a stop but the shaking gets progressively worse as I go faster.

On my way to my buddy's place, the shaking was a lot less noticeable, but the fastest I went was 30 MPH. He noticed the shaking during idle, which he said was the engine skipping, but could not find a culprit upon a visual inspection and not put up on a rack. He says it could be a ton of things, but with no check engine light or codes from the computer we don't have a starting point.

One possible thing I can think of is I fill up at the local Wal-Mart gas station, which uses a 10/90 ethanol/gas mix. Is this a bad thing for my engine?

Other than that, does anybody know where we should start looking?