Yeah.... we're getting a bunch of script kiddies banging the server. Not QUITE a DoS attack, but close to it in some cases.
In addition to that crap, something is hammering on the "recent posts" pages (recent posts is one of the most "expensive" queries in the system, are far as server resources/usages go.)
We did take a step DOWN from a dedicated server (which is what we were on before -- but it was also hugely outdated and ripe for pillaging) but, given the current network traffic on this site, the shared server we moved to (with modern equipment and software0 should handle MOST situations.