Over the past few days have noticed a slow but steady rise in my CPU temps. Taking the fan of my ThermoEngine, I found enough cat hair to make a small mat with. It might just be worth checking every so often!!
Sinks with narrow spaces for airflow will clog with dust bunnies, and cut airflow drastically. I have one of those Agilent Acticooler 's. I checked the temp recently and it was like 20F warmer than 3 months ago. The slots at the bottom, where most of the cooling is done, were completely filled and blocked. I got the dust out with a stiff 1/4 inch solder-flux brush and the temp went back to normal. It was quite hard to get the stuff out.
I've checked people's computers where the HS fan is making a godawful noise, and found the fan tangled in matted strands which made it turn very irregularly trying to drag the junk around.