Only a suggestion.
First dump one of your 8800GT's.
Install nHancer.
Disable your multicores, thus running CPU as a single core.
If that works try putting back the 2nd 8800GT.
Oook, slow down there, tiger!

I'm trying to balance my system on a knife-edge here, and so any modifying the innards is out of the question (I also have a warranty I'd rather not void).

nHancer is already installed, as well.
You must have edited your post else where would I get that info from.
Jet, I'm not joking here, anyways, leave the cards then but first make sure you have FPS set to at least 80, once the problems are all solved you can bring it down again if you want to.
FS9 does wierd things on multi processors hence the need to only use 1 core.
I have just checked my PC, if I enable the other core, I get shimmering if I look at a pole, a fence and stuff like that.
If I then set my FPS target to less than 40, fly open view (press w twice in cockpit view), in steep climb, set throttle at min, and when the plane starts to dive, watch the horizon and everything shakes and shudders very bad, set up the FPS to 80 and its smooth as silk.
Also as Sean says, check your Mip-Map although I'm sure thats the first thing you did.
I find a setting of 5 is great.