If any of your hacks use the AJAX system, such as vBShout for vB3.5, they could be your culprit. AJAX by nature, is very server intensive, because it has to make 13 separate transactions for each update to
EVERY client. This article from IBM explains this process rather well:
http://www-128.ibm.com/developerwork...=dgr-JavaMVP02
I had to remove the awesome vBShout from my site because of the loads that it was placing on our shared server, and our host kept shutting us down, because we were lagging all the other sites that were on the same server.