1. The usual. At the 1st of the month use canned air on the vents. Canned air and use are on the web so I don't write much more than this.

2. If you have not been keeping up the cleaning start now with a daily vent cleaning for a week, weekly for a month and then back to the 1st of the month. If this is the cause, it usually clears it up in a day or two.

3. If 1 and 2 don't pay off, I'd try a deep clean of the insides (you or a tech does this if you don't do your own work) along with replacement of the old heatsink compounds.