My 2005 GP GT does the same thing.... when I turn off the car and back on most of the time it will fix it. One time it was running perfect and then I stopped at the gas station to fill her up and when I started her back up and left it was acting up so I pulled over and turned her off and back on and it was fine again... When it was acting up I did take it to a transmission shop and got it scanned and 0 codes came up but they took it for a drive and said the clutch is slipping. I really don't think it was the trans slipping because I talked to my GF's dad who also owns a 05 GP told me he has done the same thing....
