Recently my 06 GXP trans has been acting up. 1st, 2nd, 3rd are great and perform just fine. Problem is somewhere in 4th at around 65-70mph it will jump in rpm's from 1800 up to abt 2300 and back down again. Just today while accelerating to pass on the highway it stayed in 4th and shuttered pretty bad. Fluid is 5000 miles old. Did complete flush (including converter) and replace filter and gasket. Used DEX 6 Valvoline full Synthetic. Any one else had these problem??