hi there mate i had the same problem and iv drove a car before like this and i put it down to the drive shafts but i wanted a seconed opinoion,so i went to my local garage and told him simptoms he said it could be your fly wheel but when he tuck it for a test drive came back put it on ramp and put it in gear and said to me yes its your drive shafts wobleing up and down when under presher, iv change them now and all seems well, please not my car is a fiat 1.9 deisel but the runing gear all works the same.