So here is a funny one I haven't run across before.
I used to have two cards in my desktop mining, but one fan started to die so I've taken it out to get it replaced, so now I am just using the one.
Before I could just put cgminer on dynamic for the card the desktop was using, and everything was fine, now I usually just close the miner when I'm gaming.
Anyway, I was sitting around watching some TV and my GPU clock kept getting dropped down to 400Mhz. No idea why. CCC couldn't change it back, Trixx could but only for a little while until it dropped down again.
I figured there might be something wrong with my motherboard, and started to look around for replacements and possible fixes.
Well, after searching around a bit and almost buying something new I stumbled across a little setting in flash player for hardware acceleration.
This never happened when I had two cards, but apparently hardware acceleration in flash player makes the card go into UVDA mode (something optimized for video rendering or something) which has a clock speed of...... you guess it, 400Mhz. Never ran across this, because my rigs obviously don't use too much flash
So, long story short, if your card keeps downclocking itself, check that first, in fact always check everything because you never know =)