Hotpoint wd420 won't complete cycle?

I have a hotpoint wd420 which won't complete a washing cycle. The machine will fill with water normally but although the drum is turning, it does'nt seem to be agitating properly. The machine will drain, and I have tried reducing & redistributing the load to no avail. When I put set it on a spin only cycle (short or fast spin), the machine begins to spin, but as it approaches spin speed, it seems to cut out and slow down again. It will do this a couple of times before stopping and failing to complete the cycle. Does anyone have any ideas on what the problem might be ?