I've noticed that while playing the game, my video card will revert back to the "powerplay" clock settings for a few seconds up to a minute. During this time the game runs at about 15 FPS and things are noticeably laggy. Has anyone else noticed this?

since the GPU usage can drop pretty low I figured the card thought it wasn't needed and defaulted back to powersaving mode.

I have a HD2900Pro 1GB and have yet to notice that in C&C3. Only time is if I have like 1K units running around blowing everything up.

What driver version are you using? Also I noticed you are using a 3870X2. I am sure there are problems with that card still that others don't have.

Cat 8.3 with the newest patch of the game short of the expansion.

Hmmm. Might just be an issue with you card, the 3870X2. Cuz I run it at max everything and res at 1280x1024 and never have that problem.

Is there any specific time it does it?

I have not been able to pinpoint any specific times, or actions to cause it. I think I notice it more towards the biginning of a game, when unit and building count is low.

So I decided to download Kane's Wrath from EA last night...
I first played a skirmish with settings on high but no AA @1920x1200 and the card would drop it's clocks from 850 to 297 every few mins which brought the game from a solid 30 FPS to 15-20 FPS. The problem seemed to be more evident. I thought, "maybe the card needs more work to do to keep the clocks up", so I turned AA up a couple notches. Solid 30 FPS until the slowdowns again. The slowdowns were longer with the expansion, taking up to a min each time.