i have a 2003 trx 300ex quad. when i go trail riding or on a motocross track,and the engine gets really hot from driving. when i click up into high 2nd, all of 3rd and some of 4th gear the engine seems like it is holding me back and bodding down. the engine sound liek it is missfiring but when i am in them certain gear i hold in the clutch the engine seem to be fine. also i noticed wen i go off a jump and press the gass in them gears the engine also seems fine. i have asked lots of people and they have no clue what goin on?

what do u think is wrong??