I've emailed about this kind of thing before.
I'm using 5Ghz with CM9s on quite a few antennas (>10) to make point to
point links. I also use 2.4Ghz with CM9s too.
Sometimes I find that I cannot, or have great difficulty, getting a
"managed" side to associate to a "master" side at 5Ghz. Ok sometimes
the signal is poor (like in the -70's) but sometimes the signal is good
(50's or 60's) and I can see the master side with an iwlist scanning and
I can see the managed side sitting on the right frequency for a while
and then just going off and sweeping through frequencies again until it
gets back around to the right essid/frequency and over and over...
Sometimes if I reboot the managed side it associates a-ok.
I have the distance set ok with athctrl and I never ever have this
problem at 2.4Ghz (so I kind of cringe when it comes to trying to make
another 5Ghz link).
I've tried lots of versions of embedded linux (Metrix pyramid and
Voyage) and I've rolled my own later versions of madwifi too - always
this same situation!
Has anyone else seen this kind of thing too?