Well to be honest
that's the only thing I can think of that MIGHT fit. The phone mounts the sd card directly after boot. (Ha... Swype just put booty... mount... booty... ok, never mind) Anyway, it's interesting that it gets as far as the Lockscreen before it happens, and works for awhile (weeks?) before it happens again.
It's just that these froyo Roms don't have a history of instability, you know?
Also, I know you don't want to go back to whatever sd card you may have lying around after being used to that 32gb, and SanDisk has a good rep (lifetime warranty still??) but how about when it happens again, try a known sd card and run a kind of minimal install for awhile just to check it out? Sd cards with sectors (well, cells or whatever) starting to go bad cause weird random shit sometimes.
edit:
I'm assuming you do a full wipe yada yada when changing Roms, and also that you've tried setting permissions after getting it setup and running...