V50 General : V50 2.0 d se **jerking **help **

[linearamp]

i have a V50 2.0d SE 2006 with only 38000 on the clock,
just recently is started to jerk at certain speeds . i wouldnt call it a misfire .(maybe a stumbling ) hard to explain.

in idle this doesnt happen ,idle is fine .it doesnt happen when im on a motorway doing between 50-70 mph in 5th or 6th gear..

Only seems to happen if i'm running around town especially doing around 30mph in 3rd gear thats when its the worse.its not cutting out / stalling or anything like that .when it starts doing it and i accelerate it stops .it doesnt do it when i take my foot of the acceleration pedal .just at a constant speed.

I have no warning lights on .i have taken it out on a open road and given it a blow out in 1st 2nd third gears (red lining every gear) i get no black / blue smoke from the exhaust .its driving me mad as my wife thinks im doing it on purpose (she doesnt drive).

Can anyone here give me any advice, apart from take it to a dealer thats going to cost a fortune ...maybe point me in the right direction as to what to check .

thanks in advance