Even after all the changes I made, drivers are stable but the graphics cards just downclock during my game.

Something in game is affecting this but doesn't make sense to me. When I do not so CPU/GPU intense stuff, both clocks stick at 626, one might downclock but usually they both stay max. When I do really intense CPU/GPU stuff, they at somepoint very soon after both downclock to 400mhz. I'm so sick of these cards I am so close to putting them up on ebay and getting a ATI.

Is there anyway to modify the clocking profiles of the cards so that when it wants to clock to the 400mhz profile it just stays at 626?

I feel your frustration dude, I have had an absolute nightmare with down clocking problems on my 2 GTX 260's.  I wish I could give you an answer, but I still have not found one.  I swear these video cards have a mind of their own.  One day things work great, the next not so much.  I can tell you that I have had alot less issues since switching to the new 196.34 beta drivers.  Have you tested with another SLi bridge?  Those things are notorious for going bad.  Have you reseated your cards?  What are the specs on your APower 800w PSU?  Does it have enough amperage on the 12V rails for 2xGTX260's?

Rivatuner won't let me change the low power 3D profile. also the 'force constant performance level" is greyed out.

idk the 191.07 drivers are soso. have you tried the latest. i have had no issues at with 196.21 drivers. ive been playing mw2 single and multi  tf2 left  for dead 2  and batman and havent had not one problem

I enabled it, I'll be able to fully test it sometime within the next couple days. I'm actually using 196.21, I forgot to change my sig. And yes i've incurred problems with alt tabbing, since I have to do that a lot when I am gaming usually. But I just change AA to x1 or x2 in the game and it resets the clocks, but regardless it will eventually downclock during play and I can't change the video settings mid-play to amp up the clock again. Hopefully Rivatuner will solve my problems.