Possible driver/codec conflicts.
Codec packs can cause a lot of unseen problems like this, until the problems crop up intermittently and annoy the crap out of you. I'd suggest starting by ensuring you have the latest drivers, then remove the k-lite codec pack. If you encounter a filetype you cannot play, use gspot to get the exact codec you need.
At first this will be a pain, but there will only be a few codecs you actually need is what you should find before long - and random issues like you are seeing may no longer occur.
Could also be a physical problem, loose wires or something - but if restarting the software fixes it, then its more likely you are running into a software conflict with drivers/codecs.