Vauxhall Corsa weird revving and half powered pulling off

Hi,
My Corsa has been having issues with over-revving for a while now. It started off just revving a little high after starting, but now if you stop at a junction before the engine is warm it sometimes revs high then drops off over and over. The last time it did this it settled at about 20,000 after i revved it up to 30,000.
Also, when pulling away in first gear it often doesn't seem to give full power for a few seconds (this may also be affecting 2nd and 3rd gear, but I'm not sure as the difference is less marked...)
I plan to try and clean the idle air valve to see if this will help with the former problem, but I'm wondering if there's anything else that might be going on that I should get looked at.
Any suggestions gratefully received!