When I am driving along my car starts over-revving up to 4000 RPM randomly. I've carried out a slippage test and there is no clutch slippage. Any ideas guys?
Frustrating! If it's happening intermittently then there's a few things it could be:
Oil leaking into the clutch casing
This could be coming from various places - but if you get any oil on the friction plate you'll get temporary slip from the clutch until it burns off.
Possible.. but I think you'd be seeing more consistent symptoms then you are.
Misaligned/sticking clutch bearing
If you're seeing the symptoms happen after you've changed gear then the clutch bearing could be at fault - if it's not doing it's job properly the friction plate might not be settling back in contact properly. Have a listen to see if you can hear any growling or rattling sounds coming from the transmission when the symptoms occur.
Again this would only be happening when you change gears, and you'd usually feel some vibration or sticking when moving the pedal.
Broken friction plate springs
This is incredibly rare - but it does cause symptoms remarkably like what you describe:
Basically when the bits of spring come away they get trapped against the friction material and cause it to intermittently drag.