clutch problem

I have a clutch problem on my 2003 R6.I have had the bike for the last 3 years and changing gear has always been a bit notchy but nothing worth worrying about, but the other day when i went to take it for a blast,I could'nt get it to go into 4th.Eventually it did and 5th & 6th were no problem but on the way back down the box it would'nt go into 3rd.It stayed like this so i gave up and headed for home.I've since stripped the clutch down and checked everything with a vernier and all is o.k. so i have put it all back with lots of lubrication.

I took it for a spin and everything was fine, in fact better than it had ever been-the gearchange was really smooth (the way it should be).Well it was untill it got hot,then the same old problem returned.

Any ideas please?

I will add,and i don't know if it's coincidence,but i have recently changed the oil and filter (using a genuine filter and comp 4 oil)