Random Over-revving Golf

[ButchGTI]

Driving down the M6 yesterday we stopped at the toll booth. We accelerated away in my friends Golf, Mk3 1.8 petrol (not gti). At around 4000 rpm the revs flew up by 1000 without a surge in power. This happened in 3rd and 4th gear on a few occassions throughout the rest of the journey. There was also a random rattling noise coming from the engine bay.
Any ideas what the problem is?

[lanciachris]

Clutch / Clutch release bearing.