it'll be the clutch switch for definite. Mine does it because I haven't fitted the stopper on the clutch pedal since fixing the clutch pedal. It's ok at low revs but if you change up at 5000rpm or higher, you do see the revs climb by 500 - 1000rpm with the clutch depressed.
I would have thought there would be a rev limiter which will stop you from over revving the engine.
Check the clutch pedal, there should be a piece of grey plastic that clips onto it that activates the clutch switch, it will either be missing or the switch will be knackered. If the plastic is missing, it costs about £2 from dealers.