gambo08

Hi all i have a 2.0 mondeo estate a bought for doin my house up,the only problem with the car is wen driveing along in 4th gear without any pressure on the throttle the car jumps as if its missing wen u aply throttle its fine and only does it between 40/50mph ,ive fitted new plugs and leads but still no diffrence any ideas please help cheers.