i had that pprob in another car and it was an engine mount, when pulling off and in a high gear with alot of the engines power lifting the engine up was slighly pulling the driveshaft out and makin the car jump, generally something that is a missfiring problem but i wouldnt rule this out, had my head done in for weeks as it didnt always happen like you said, i only realised when it got really bad as when you go round a corner it gets worse