I'd go with the gummed up lube theory for starters. Yes, as stated before, WD- 40 squirted in the hub through the filler cap and the chain hole in the axel. It's messy as hell but it does work. Be very generous with the WD-40. You will have to degrease the wheel and brakes but it can get the hub loosened up to where you do not have to take it apart. Either carefully ride it or suspend it and crank by hand . Shift a lot.
If this works, adjust the chain and lube as recommended.