user manager famous problem

Hi,
I have a problem with user manager, and as I noticed it is a common problem, with most of the routerboards that I used.
Now, I am using CCR1036 version 6.30, I have about 4000 user, about 250 active.
The problem is after some time, users get the message “RADIUS server not responding”, and the user manager is becoming so slow, I have to wait alot of time to create one user. so I reboot the routerboard, but I have to wait a long time to user manager start working, I have to reboot several times, I tried to rebuild database, I make the user manager better for few time then the problem return, it occurs almost every day.
I disabled log.
My database size is about 100 MB, In use 100%, Free disk space 612 MB. I deleted about 1500 user It is now in use 95%.
I believe that there is a problem in user mananger.
A friend told me that there is a way to solve all of these problems. I don’t know.
I need you help.
Thanks in advance.

Please, it is a serious problem,. and it is not only with my routerboard..
I am waiting.