Your timeout should be plenty. If it does not interfere with anything else you have running, try 20 minutes on the interim interval. To me, it is better not to get updates as often and freeing up the radius server to handle logins.
Sounds like you need more power somewhere.
The only problem I can see using 20 minutos would be my IPPool (SQLIPPool) witch I should rise it time to something over 20 minutes. My most valuable resource is my IPV4 address range and I need it to be dynamic and fast release when it is not been used.
As I read your message something occured me: I will only have a problem with PPPoE when I got a stalled session and, if this happen, I will need to phisicaly solve the problem most probably dropping connections over a NAS or something like that.
I will try to update this time to something about 20 minutes.
About the power somewhere we are running this over a dual Xeon 2.8 Ghz in a Vmware enviroment. I can set it up to 32 Gb of RAM but nothing of that solved the problem. Looking into MySQL I can't see CPU or Memory going up. It seen to me that MySQL is the problem for some reason it can not handle that much conections.