I would run Dpc with sonar runing playing one of the demo songs on it let it play to the end of the track then let us know the readings.
Restart the DPC checker then run it again with windows siting doing nothing for a couple of mins.
Tell us your findings from both.
It could be things like do you have a lan card on your computer ? have you got blue tooth on your computer ?
if so it needs to be disabled!
Things like that can make a huge difference and Dpc will tell us if it could be something like that causeing our pops and clicks mine was :)
Audio Computers need 2 hard drives one for audio one for the OS Sonar. Both drives also need to be at 72.000RPM at least for audio.
I am not saying that the hard drives are the problem its more likley to be something else runing in the background so run dpc and see what the readings are first so we can see if your computer has problems that need fixed before you even change anything in sonar :)
Norrie