I'm wondering if this problem has anything to do with attitude. With some aircraft you can get the nose too high, to the point where the antennae can't see down far enough, so the ILS loses it's capture.

Has anyone tried flying the ILS at a higher speed?

Mark, I too use YMML and YBBN with all the FTX areas and I saw a guy on their forums post up some figures on GTX470 and 480. My 5870 was around the same fps as his 480 but at a higher resolution. My 1920x1200 vs his 1680x1050. They are very good as long as you use Jesus Altuves SM3 mod. (That stops the stuttering in heavy cloud) The ATI have a bad rep in the FSX world for some reason but I can assure you they work as well if not better than a 480. Jesus Altuve himself said he regretted selling his 5870 after he discovered the mod.
The worst I get is mid teens at YMML in the 767 vc and that's usually when there's a lot of traffic. (My Traffic 2010)
So if cash is an issue then I can highly recommend the 5870.

It amazes me how far and wide FUD can spread and the fanboys who spread it should be ashamed. We are supposed to be helping each other out, or so I thought. (I should mention that wasn't aimed at anyone here)

/Rant.