sounds like the slipper to me. W/ gear cover off, on the gound, see if the the large spur (1st gear) spins under throttle. If so, then it's the output shaft in the tranny. I sheared one leading a RC ProSeries race!!

tighten the slipper all the way and see what happens, make sure all the spacers, slipper pads, brass bushings, etc. are all seated properly. Hope this helps.