I have to reboot my web server regulary because it fails to handle new apache requests. Please check the graph - workers are still raising. But I am not able to figure out why? What can cause this problem?

Feel free to check my monitorix stats: http://jsemtu.cz/monitorix/

My mpm_prefork:

StartServers             5
MinSpareServers       10
MaxSpareServers      30
MaxRequestWorkers     70
ServerLimit   70
MaxClients  70
MaxConnectionsPerChild   180