Polo 1.4 timing out

Hi, My mother in law's '96 1.4 Polo is running really poor at the moment. The problem started last time it was serviced. It came back from the garage and was running really bad, making a very loud thumping noise as the engine accelerated or driven uphill. I took it back to the garage and they told me it was an electrical problem and not something they could sort out (last time i go there as they caused the problem). I took it to my local Seat garage as they look after my car and they told me it was the timing. They bought the car back, much better than it was but just not quite right. Ive not approached them since as they are quite expensive and tend to charge by the minute !!
Does anyone know or have any ideas how I can get this sorted ? Was such a good, reliable car before this and has only done 70k so loads of life left in it yet.

Thanks
Richard