Also... just realised my gearbox was also a diesel one and that used to pop out of 3rd and whine like a bitch in first. I found the easiest way to test for knackered gearbox mountings is by getting a thick bar and wedging it between the gearbox and the gearbox crossmember and pushing down... i.e trying to lift the gearbox up with leavarage. If it has movement then you know its the mountings..... Or if ur lucky it could just be that the nuts have come loose. But very unlikely.

Graham