or could just be goofy drivers, try diff ones.
uninstall divx and try (dont know the details or if its been fixed) but some codecs were affecting some video drivers. ive experienced strange lag before, and uninstalling divx did actually help. this was for some oblivion problems i had last year.
try playing blockland and them games and all lowest settings and res. see if you still have lag.
open up your custom video settings from the desktop and reset all them stuff to default and try.
