[Serusers] SER stops responding

I have experienced (for the second time) a situation where SER (0.9.1) has stopped processing all register requests. Note I am also running mediaproxy 1.3.0. TOP shows 4 ser processes running each using between 20-30% CPU. There is one zombie process which is shown as [SER] <defunct> when I do a ps ax. There are only 5 running processes (4 SER + top). This is a test server that is not doing anything else and I am simply trying to register 1 UA against it. The server was running fine for a few days and then suddenly went AWOL. I looked back through the log and didn't find anything untoward. Any suggestions on what's the best way to try and find out what's causing it? Killing SER and starting again worked but I'd rather try to find out what went wrong. Maybe these are the pleasures of using a non-stable version? Regards Cameron