Spin cycle start/stop after motor replacement

I had to recently change the motor on my Hoover DXA 68W3/1-80 (31006738), all seemed ok apart from the pulley on the motor looked smaller diameter than the original. I was advised by the supplier that it was the correct model and fitted it successfully. Everything was looking good in the first wash until it got to the spin cycle, the spin speed was set to 1600 and as soon as it started it seemed to stop and then start again and again and again. It is almost as if it hits a max speed limiter that shuts the motor down but as soon as the speed drops a little it starts again. I have also tried running at different spin speeds but to no avail.

I have asked for advice from the supplier and they have suggested just to return the motor, however this will leave us without a washing machine again until we have a replacement.

Can someone please help, if it is likely to be a fault with the motor then at least I can send it back and hopefully get a replacement, but if it something else I will just be wasting time and money. Also will the slight difference in the size of the pulley make a difference (only a couple of 2mm).

Thanks in advance
Andy H