Nissan Almera loosing power?

My car has recently started loosing power, but today I had to over rev it to change gear or it jerked you forward violently. It did this last year but not as servere, so I put it into the Nissan dealership for a service and extra which cost me over £600. The problem was still there but not as noticeable and when asked they said they couldnt resolve it. Does anyone know what it could maybe be, or maybe what I could try myself as I'm skint now and need the car badly for my job. Its only done 69,000 miles, has recently had new brakes, starter motor/sensors, back box and clutch. Timing belt was fine also.