bojan.u

I recently bought an R 320cdi...the car runs ok, the gearbox shifts ok, but when I stand still and put it in R or P, the revs drop a bit and it wants to go forward or back..... if I leave it on D and press brake the gearbox tries to pull foward very hard and and the engine struggles to help gearbox... Before I have bmw and the same situation it was different...
any idea what it could be?