Freewheel slippage

[aitch]

The freewheel of my 10-speed Bontrager hub slips now and then.My bike shop has changed the freehub on two occasions.They do not know what is causing the problem and now think it is something else other than the free hub.
Comments welcomed, please.
Aitch

[DaveM]

Does it slip only under heavy load? Especially in certain gears? If so, the mostly likely cause is a worn cassette and/or chain. But it is odd that your shop would swap the freehub twice. Freehubs typically last a long time and the cassette is usually the first place you would look to fix this. You might want to ask them why they're changing the freehub. Hopefully the answer either reveals something important (like there's a known problem on that hub) or that they're dufuses. Did they charge you for those repairs?

[aitch]

hi DaveM, thanks for your response.The loss of traction occurs usually,after freewheeling.The cassette and chain are both new.The shop charged me for the first change but not for the second,saying Trek had agreed to a one off replacement.Another shop has told me may be due to using grease rather than oil and also on the strength the springs pushing out the pawls.
I will keep you posted.
Aitch.