2007 Vauxhall Astra lack of power

there seems to be a problem when accelerating normal when hitting 3,000 rpm in the higher gears 4th,5th and 6th. it only happens for around 500- 1,000 rpm then its back to normal, to be describe the problem is like the engine jurks when cold and warm. there is no lights showing on the dash either. I have taken it to a number of dealers it shows up nothing on the computer, but there is some sort of problem but a annoying problem to be said when trying to overtake as it just holds at speed when accelerating. When accelerating hard it seems to not do it or put the sport button on it doesn't do it either. could some-one please fill me in with a few suggestions as it is becoming a concern and problem.