I know this isn't a support channel, but Netapp's not been useful at all, so what the heck:
I have an F630 that's run well enough for five months. On Sunday we upgraded our filer to 5.1.2 from 5.0.1d4 at the suggestion of our Netapp SE.
Just curious - instead of going to the trouble of dowgrading - have you considered a quick upgrade to 5.1.2D2?
It's available on the Now site - and there is a substantial list of bugs it fixes.
Graham