1999 Opel Astra Jerking through the gears

Engine problem
1999 Opel Astra Engine Size unknown Front Wheel Drive Manual 146000 miles

Hi there,

I'd be very grateful if you could shed some light on my problem before I take it upto the garage. Firstly I drive a 1.4 16V Opel Astra, Whats happening is that when your travelling in gear from 2nd to 5th between 15 - 35+ Mph without pressing or lifting from the accelorator the car will start to jerk as if it was in the wrong gear for the speed it's doing, which it isn't? Is this a fuel problem or a dirty sensor or something, would this be an expensive problem to fix? Does it do any harm to teh enginge the longer I leave it to fix it?

Many Thanks

James McIlroy