Have a 2007 Ram 1500 MegaCab RWD Hemi. 83K miles in Excellent Condition. Recently experiencing a transmission issue. After accelerating and when transmission finally shifts to highest gear and RPM drops, am getting hard slippage or surging on level or declined roadway between 20-30 MPH. As soon as you increase RPM it works fine and works and shifts fine at all other speeds and under load. Is this a gear problem or a sensor problem or what? Is it best to go to a dealer or AAMCO to have problem diagnosed? Thanks for any help!