Hi,
This sounds like a weak spark under load, or possibly bad carburetion.
I'll just list some of the usual causes/cures for this problem:
Start with:
NEW spark plug(s)
NEW Spark plug wires and caps
NEW or washed airfilter
Check intake manifolds/hoses for leaks, ANY sign of cracking replace.
NEW needles and needle jets - should be replaced every 25-50,000 km anyway, as they do wear and cause slightly rich mixture/bad fuel mileage. Your improvement from lowering the needle suggests that this may be a good place to start. "...Would that mean that something else causes the problem? Leaking inlet duct, worn carburettor, etc..."
Yes. The only time you should change the needle setting in the normal life of the bike would be if you modified the engine/exhaust etc, or were at extreme altitude. If just changing the jetting/needle position etc cures the problem, you haven't found the REAL problem yet.
Are you SURE all the passages inside the carburetor are clean? They can be very difficult, you need a powerful cleaner to do the job.
If all that fails, you may need to take it into a shop and have the coils and related items load tested. They may be fine in a static test, but under load break down.
Hope that helps, good luck!
------------------
Grant Johnson
Share the Dream!
at:
www.HorizonsUnlimited.com