|Hopefully someone more experienced will come along shortly as 'could' be more serious power issue.|
In the meantime, I had similar problems with an older PC around 10 years ago. If you haven't already tried then look here >Start>Control Panel>Display>Settings and see what you have the Screen Resolution set to.
Make a note of the setting so you can put it back that way if necessary and see what it performs like if you increase or decrease (left or right slider on Win XP Home). Remember to click 'Apply' for it to take effect. It worked for me but doesn't always follow.
Hope this helps.