A : Clean your computer.
Explanation :
After a little trouble when I started folding A3 WUs on my stock Q6600, I quickly got 3200 to 3500 PPD (execpt for p6013).
About a month ago, it started going down slowly, bit by bit.
Then 2 weeks ago, I got a big drop to around 2400 PPD.
One week ago, I got my i7-980x and wanting to overclock it I installed temperature monitoring software, and while I was at it I installed it on my good old Q6600.
Revelation, It was at 90C on two cores and throtling back now and then (explaning the drop in PPD).
I opened it up and it was filled with dust and cat hair.
I cleaned it up and it's now at 82C and my two latest A3's where at 3698 and 3891PPD. WOW !
Clean it up and increase your PPD !