xvid, WMP, VLC players all stutter? any help?

i have 4 gigs dual channel, 3.4 gig hyperthreaded cpu, and a 512mb graphics card (drivers updated). when i play anything on windows media player or vlc player it stutters every few minutes and plays fine for a while and it does it again later. it's very annoying. all the codecs are up to date and everything. sometimes flash wont load from youtube, vimeo etc. (it load but doesn't play but a few seconds and freezes)

does anyone know what could be going on? could it be my motherboard? the ram is brand new.