RE: Troubleshooting somewere?!
Does the engine speed increase when it starts to descend? It could be your one way bearing in the counter gear that is slipping which will cause lack of drive to the main rotor shaft. Otherwise it could be the clutch lining needs replacement. Test the one way bearing by holding the rotor with one hand and try spinning the counter shaft (driven from the clutch bell via the belt) with your fingers and it should only spin in one direction. If it locks correctly in one direction then put more finger pressure on it to see if it slips. It is a common problem with the one way bearing. If you want to replace it I suggest finding the upgrade one way bearing from Kyosho as it has more contact points to provide a better grip of the shaft.