OK, I'll look into all that. I appreciate the pointers, thanks.
Oddly, the problem does turn out to be MediaWiki-specific after all. If go to my other urls after I've been to my media wiki, they fail sometimes. Something is dumping core in my MediaWiki directory after a browse there, too.
But if I leave it a while, and avoid media wiki altogether, everything's fine (and quite a bit quicker than before). So I'll have to look around to see if I can find any eAccelerator/MediaWiki-specific info.
Anyway, I've spent as much time on this today as I dare. Cheers.