I've now had a handful of rides on this 928 I bought, and probably once on each ride it's missed a gear, clunking and rattling before going in.

Is it a common problem and should I be worried?

 

My 600F does this if you try to go into 6th when you're not really going for it. I've also had it happen on the 929 on one occasion. Generally I think it's because on occasion I don't really move my foot enough. Do you roll off the throttle when you shift up and blip when you go down ? Is your shifter adjusted for your boot ? A few mm of adjustment (lever down a few mm for me) helps a lot.