I have a 2007 gt with a built 4.6 steel stroker crank manley rods diamond racing pistons. Stock cams, top and bottom end stud kits. 76mm hellion single turbo kit with methanol spray. Made 650 rw hp and 686 rw tq on 92. My problem is after tune car ran fine then over the next 150 miles started hanging rpm. Only when moving. Runs perfect if sitting still but if moving it hangs if you push in clutch and rev it will hang around 3000 or more and very slowly come down like count to 5 or 10. If you shift it will hang or even raise up. Tuner says it's not the tune but I'm at a loss. Oh yeah its running a gt 500 throttle body and 1050x injectors.