just bought a 2001 GT, and it recently started to hesitate and bog down for a few seconds while under aceleration, then it clears up. I then noticed while sitting at a light my tach would go up to 1000 rpm's and then fall back to 700 rpm's. But if I give it some gas it clears up. It seems I have a bad plug or something. ANY Ideas or thoughts what it could be? HELP!