N5010 System lag/audio "static"

Hi, I'm using an inspiron N5010 with windows 7 and for the past couple of months I've been getting a sort of system lag, the mouse moves jerkily, windows don't load smoothly and (most irritatingly) music starts to crackle.

The problem isn't constant and doesn't seem to depend on the number of programs open (it happens randomly even if i've only got something like word open), internet browser or program being used.

I had a boot problem in the first 2 weeks of having the laptop (it's only about 6 months old) and got the motherboard/fan replaced so I don't think it's a problem with the system overheating, there's 350GB hard-drive free and 4GB RAM so I wouldn't expect a memory problem, but CPU usage jumps to 60-90% when the crackling starts.

Any ideas???