The default value of 8M was too small. In fact, to import my MovableType content, I ended up setting it to 128M, I think.
memory_limit = 16M ; Maximum amount of memory a script may consume (8MB)
and I diddled these values a bit, though I suspect the change to php.ini is where the battle was won.
StartServers 25
MinSpareServers 5
MaxSpareServers 10
MaxClients 75
MaxRequestsPerChild 0
With almost 4400 page views by 20:30 today — a Saturday — and no load-related weirdness, things are looking better.