Hi! It may well be that the variable speed "pot" (rotary control) needs cleaning. I would recommend giving it a spray with "Servisol Super 10" contact cleaner/lubricant. You would find this in most electronics shops (Maplin or Radio Shack for example) or on eBay. WD40 may also help here. Simply remove the knob from the shaft (if possible) then spray the shaft. Leave it for a moment to penetrate, then slowly rotate the shaft through its' full travel a few times. Repeat this a few times, replace the knob & fire up the saw.
THE CLEANING PART MUST BE DONE WITH THE SAW TURNED OFF!!!!
I hope this helps...it's sure cheaper than buying a replacement part!
Keith. (Norfolk. England)