Awhile back I went from 4Gb to 8 in my rig. Not long later I noticed my speed had dropped from the normal 3.2GHz to 1.6 although all I'd done was add the ram! Wasn't doing anything cpu-intensive so I left it alone without a problem. Recently when I re-installed W7 I decided to investigate and fiddled around with the BIOS. OK, the cpu multiplier was  and after some calculation I changed it to 17 which gave me 3.3GHz. Great! No! Random freezes! Ok fine I'll settle for 3.2, NO! 3.0? NO!
1.8?!? NO!. Finally chose [Load Optimized Defaults] so the BIOS could decide and guess what? 1.6!
Why would adding RAM make the board want to slow down?! Do I need to increase the core voltage or fiddle with some other setting?
The System: Homebuilt
Mobo FoxConn A79A-S
CPU: AMD Phenom II 940 3.2GHz (formerly)
RAM:8GB Hynix PC2 6400U DDR2-800 (2GBx4)
Graphics Card: GeForce8800 GTS
Voltages (from BIOS screen)
CPU PLL 2.49v
I don't know why Cool & Quiet keeps getting mentioned IT'S NOT ON. The cpu speed doesn't vary, it's 1.6GHz all the time. It was 3.2GHz before I added the ram, it's now 1.6GHz. I made NO OTHER CHANGES. Even under a heavy load (like Lightwave rendering) the speed stays exactly 1.6GHz. I can't even increase it to 1.8GHz without it freezing on me!
OK, figured it out. It WAS heat. I suspected that the insides needed to be blown out and planned on taking it to work to do so, and forgot. So, I climbed inside with a vacuum cleaner hose to do it myself. Found that one cooling fan had 2 broken blades, another was set to 'low' and only jerked once a second or so. Pulled the cpu cooler and found the thermal paste was completely dried out; acting more like an insulator. So, replaced the broken fan, turned the other one to 'med' (which got it moving), pulled the cooler and scrubbed the surfaces clean. Unfortunately out of thermal paste but ordered some. Threw it back together, turned the CPU ratio to 14 (3.0GHz) and voila. She rides! My main clue was when i ran Sandra it warned that my core voltage was too high. Of course it is since it was set for 3.2GHz and I'm only running 1.6. It also said my cpu temp was 68C which is NOT what it says on my front panel temp display. So all good. Thanks for the replies, guys...