hi can anyone help me only had the car just over a week and when on moterways etc when i hit 4grand in 4th it just sits there and wont move above 70mph then back off the throttle abit and it starts to move up slowly the needle dropping up and down on the revs but the speedo claiming up and 5th just just move i got 104 in 4th then 5th just held it there any idears?