Buy a can of compressed air, open up your case, and dust everything. Make sure to spend extra time declogging the cpu and case fans. Also, check to make sure both fans are actually working.

There could be any number of factors and sadly enough, no real way to pinpoint the cause until something in your machine dies. Until then, the most likely culprits are:

a) Overheating due to lack of proper ventilaton
b) Metallic grit causing a minor short
c) RAM dying
d) CPU dying
e) Virus

The dusting will take care of a and b, you've already scanned for e, and you'll just have to see if c or d develop.