my rear wheel will sometimes get stuck going in the forward motion. when this happens it will go in the backwards motion just fine but not forward? this happens while motor is running or turned off. After pushing it forward it will sometimes release and drive fine for a while and then the problem repeats? I'm new to the moped world and bought a 2004 Tomos lx for my son and this problem just started a couple days ago. Any help would be gratefully appreciated.