Users cannot login with AFP share on OSX server

We have a OD environment with 2 OD servers OSX10.6.8, 64 bit Kernel, Xserv Quad Core intel Xeon 2.26 GHz with 6 GB RAM and 1 Macmini server at another City running OD replica. We have 180 iMacs and 2500 users. After users login they get a AFP homefolder 512 MB on one of the two OD servers (internal RAID HDD, 1TB).

The homefolders are shared between the two servers. Max Threads are set to 1500 (http://support.apple.com/kb/HT3854#).

Once a week users are not able to login and get a message: u are not able to login at this moment and the connected users expirience a very slow connection not workable.

Server activity monitor shows: AppleFileServer threads 1503 and CPU load between 100% and 130% and stays there even if the number of users decreases. User cache information is set to a local tmp folder and Spotlight is turned off for the shared homefolders on the Server.

Stopping AFP en restarting it wil Fix the problem but i want to find the problem.

Does anybody have the same problems or knows how to fix it?