If windows NT or 2k or XP then check the task manager to see if it's screwing the CPU (in which case your hardware needs upgrading - memory is the likely cause, or possibly a slow/old CPU).
Also, when you say playback, are you talking about music which you've copied from a CD? if yes then this happens to me too and is down to Version9 being very new - it's only been released a few months so don't expect it to work properly for a long while yet - keep checking for upgrades, plugins etc.. (as i am resigned to doing!)