Sound degrades to noise

On inspection of any soloed track, its playing back time stretched and pitch shifted, like its on the wrong clock, but it isnt (24/44.1 both project and FF).

This is in reaper 64, Win 7 64, fast pc thats very unlikely to be the problem (6g of ram, cpu use never goes over about 13%)

Music playback is fine. So was this project (the first I've done with the FF) for quite a while - it was only after a little mixing and adding effects that it started (again, must stress - not many; there are 20 in total over all the instruments in an 8 minute track).

Increasing the buffer size helps a little, but makes latency unacceptable.

Any help much appreciated, I'm rather lost on this.