I am really not sure if it is a DDOS attack or not. What I do know is that I upgraded to 4.10 PL2 on Thursday and the problems started shortly thereafter.
The actual problem is that a large database request locks the post table and prevents any other requests from running, letting the max connections get filled up and crashing the database.
It happens in no discernable pattern or timetable.
I am working with vBulletin and with a consultant to try and resolve the issue.
Thanks for being patient with me.