Give it a try and turn it back if it don't work for you (it depends on your disk swap).
You seem to have checked the other things suggested in this thread.
Remember all types of lag stacks, so checking all is worth the time, like sound drivers, etc.
If you don't want to run in low graphics you may need or want a newer card.
On my newer computer i found that a not-so-new radeon like the 63xx can do most of the dirty work.
(cheaper than the newer ones)