For starters...Ill try to help.
How much ram do you have?
What is your video card and are you using the latest drivers? (This issue is usually related to your hardware and/or drivers).
Also check and see what your virtual ram is set at. It should be double or more of your current ram (this is a suggestion and others may have better advice) I say double so if you have 4 gigs of ram, double your virtual memory to 8 gigs.
See if those are covered and lets hear what happened.