Well, it seems that running Yaki under Psyco is something to be avoided for the moment.
You see, Psyco bloated it up to 600MB after a few hours, apparently due to the way full-text indexing works.
Given that Yaki has been running for several days now without any hitches until I enabled Psyco yesterday, the best fix was to disable it.
Just in case, however, I enabled gc.set_debug() instead.
Shame, though, full-text indexing was blazing fast under Psyco. I have to look into this again later and try doing spot optimizations.