Sounds like the jackshaft gear is hanging up on the clutch starter ring. Could be due to a work jackshaft, gear, bendix, primary cover bushing or any of the above combined.
Check all electrical connection regarding the battery, ground strap and starter as well.
Also....make sure the starter bolts are tight, and starter to inner primary union is clean, because that's where the starter gets its ground from.....
“When the going gets weird, the weird turn professional....” ~ Hunter S. Thompson~
R.I.P. Rubberdown. 1971-2012