Correction Outlander: If it is running windows 8, take the install CD. Run it through an industrial class shreder. Light the peices on fire. Throw the burnt ash in an incinerator. Throw THOSE ashes into sulfuric acid, then drop the acid into a volcano.
After you do that, fully wipe your hard drive and then install windows 7. Windows 8 is stupid.
I suppose it might work better for some people, but I tried it and it was terrible. Harder to do everything.
That aside, it probably wouldn't cause a sudden drop in frame rate.
The dust thing is more likely. It could have suddenly gummed up the fan.
Check your temperatures!