Gearbox/clutch problem?

I have had trouble with either 3rd or 4th gear, happened 3 laps in a row during a race, just wouldn't change up. even using the clutch. Same place each time, just about got rammed from behind. Weird, and no trouble since(fingers crossed). No Idea why, welcome your thoughts.

Thanks for the replys guys -

Orpheus - i have no other problems apart from a very clunky change from neutral into 1st when you are about to pull away! Oil level is always kept at the halfway point in the sight tube!

Cat - The oil jet was cleaned only 300-400miles ago by myself, i tend to clean it evey other oil change and i change oil after evry 5-6 trackdays!

Wellyktm - so are you saying you normally do clutchless upshifts? that was going to be my next experiment to see if i could ride round it when it does happen! mine is always 4th to 5th gear! and i no what you mean about being rammed from behind!

Ill do a bit more research and see if i can dig up anymore info!