What is wrong with my BEKO WMA765 ?

I have a beko wma 765s with a peculiar problem. The cycle runs through as normal and the drum rotates at the appropriate times, that is until it reaches the spin cycle. Everything starts off ok, it increases in speed up to, i would say 800rpm for a few minutes, then stops as if the power as been turned of (but it hasnt and the front display remains lit), then just as the drum is about to stop rotating it burst back up to full revs, not gently, but all at once, which makes the belt scream and the machine bounce!! It then spins for about 30 seconds and does it all over again, and again until the time runs out, without ever reaching top speed of 1200 or 1600 (cycle dependant) I replaced the carbon brushes as the old ones kept sticking in and were pitted but it made no difference. When the motor is spinning, it is very smooth and quiet, with no noticable sparking on the comm, until it suddenly sets off at full speed which makes the motor spark, belt scream, machine jump, and me sh*t myself!! someone please help. The machine drains as normal and the pump has no problems.