Since you guys seem to have multiple servers, it came to my mind why dont you have a signup server, and the hosting one, everyone who sings up is automatically put into the signup server. once a site has been up for lets say 1-2months with reasonable amount of traffic. it is moved to the next one, so this way the new users who register non stop wont affect their service, its just an idea since you seem to be having problems with mysql constantly. it might not be practical but i dont know the status of what you have/dont have
