I said spin the gears so the two points are not in contact. Could be warped spur, pebble damage ect. The clutch behind the clutchbell, I know of two types. One is a spring that goes all the way around. If its not connected or droopy, its an issue.
If its a metal pressure spring, look for loose clutch pads that move easily, or you may hsve some good tight ones and some getting looser going bad.
The 18ss was I kit I'm pretty sure. It had HD machined steel ring and pinion vs cast bs nitro rs4 ones. It was also a lower ratio on the ss. The new rs4 3 evo became the 18ss as an rtr with a 3.0, from the t15 and the 15fe. The 18ss I want to say had a different large (.18-.20) motor while the rtrs had the .15 at the time.
Rs4 3, rs4 3 evo, and 18ss use the same parts. The older belts shared thier own. The drift has different arms, shorter arm shafts, the center shafts are different due to the parts getting moved forward for a drift biased setup.