What happens is this: starting off from rest in 1st gear, I'll add some throttle and let the clutch out and sometimes - like 1 time in 20 - the bike lurches forward unexpectedly. The only way to bring it under control is to disengage the clutch, let the revs die down and start again. Can happen two or three times in a row. And then just once last night it happened going from 1st to 2nd - just a surge and bump and then it settled down. Pretty sure it happens a lot more when the bike's cold.
Looking back, I think this started some while back. At first I put it down to my clumsy clutch action, but it's been getting a lot worse these last couple of weeks, and now it's clear that the bike's doing it, not me.
I'm a complete numpty when it comes to spanners, but, could this be the wrong oil (oil change was 4k miles ago, done by the indy guy up the road)? Or what else should I worry about?