Doubt it's the ISCV but do the mentioned anyway as it's free, that is only needed if the ISCV is jamming, the fact your revs are going up and down shows something is telling it to do so.

I would think it's perhaps the VSS, again easy to test simply drive along at around 10MPH and drop into neutral AND release the clutch, the revs should raise to around 1200 RPM, if it does this it shows it to be working.

Martin