Vauxhall Corsa jolting

i bought my car about 8 months ago and a few months later it started jolting and only seems to be when im driving at a constant speed, not when im speeding up or slowing down. i have had a full service and changed the o2 sensor in the exhaust manifold and the problem is still there. recently tried to drive it and almost lost all power even though my accelarator was nearly at the floor the car would hardly move. anyone got any suggestions please help