DNA 125 knocking and backfiring

[Dup]

Ok... its been doing this for ages:

It starts up ok, no problems and i ride off. I mainly run the country runs which have many a bump and hill etc.

Well at certain points during my journey. Mostly, not every day it does, the engine seems to cut off, like i've hit the break and then if i give the trottle some stick it sometimes picks up and lets out a big pop out the exaugst and then i can carry on again.

Its very strange. I have had it looked at by a Gilera dealer but they found nothing. I had them service it anyway and replace the rubber. Ran great for a while but now its doing it again.

Any ideas so i can get it looked at properly. I was thinking the clutch. It usually happens on an incline or after a hrd bump.