Hi there mate, I have a sherco 2.9 2002 which lucky for me has had very few problems... But, I know this may sound odd... Have you checked the air filter? Because on one of my gassers I have had similar problems with my air filter, I would also check that you haven't cross threaded the spark plug when inserting it... Oh and also make sure the rad has a decent amount of coolant in it. Its hard to say what the problem is whithout actually seeing the bike itself, so maybe have a ring around a few shops (haven trials sport) for example because they deal with sherco. p.s. it could just be that dirt has got clogged up where you insert the spark plug...