All the above are good sugestions, first would be to check the tyres are not slipping on the rims and back your slipper clutch off so that its slipping and then slowly adjust it up 1/8 of a turn untill you get it where it bights resonably quick but still slips for the first foot or two from take off, I tighten mine till it binds then back it off one full turn and adjust from there once you get close to perfect slowly tighten it up a mm or two as I have found there is a fine line between slipping and too much bight. I had a simular problem years ago and it was a result of chipping a tooth on the diff gears (sun gears). If this was the case tho I would suspect you would hear a grinding sort of noise, plus feel it when turning the rear wheels by hand. I also regularly clean the clutch bell out with a bit of fine sandpaper. Also check the friction pins in the slipper clutch as once these wear so far no matter how much you adjust it it will either slip against the gear totally or bight totally. Doesnt hurt to run an extra set of friction pins either filling every hole rather then every second hole.