GUI management is going down about five minutes after restart th eunit

A restart being a full box reboot?

Have you tried just restarting the http and tomcat processes?

bigstart restart httpd 
bigstart restart tomcat

Have you tried increasing the maximum number of concurrent connections?

System > Preferences 

I have tried restarting httpd and tomcat. Did not helped. I found some article that it can be memory leaking in this version (11.3.0 HF3).

I thin I will try clean install...

Shame. If you can that would be ideal.

I will let You know about progress.