my speedfight acceleration?

i took my 98 speedfight out for a spin and i was going down a hill and suddenly the engine lost all of its power the engine was still running and when you gave it throttle it started to move forwards but very slowly so i started pushing the bike home and the hill was very steap so i used the engine to get it up there then half way up the power came back and it was fyn, a week later the same thing happened. however the belt has not snapped and is in good condition so i thought it must be the rollers could anyone help and solve my dilemma.