Answer for El patron and Prestashark.
Yes when there are few people server works fine because as like mohitsareen server is dual 4 core. It can eat 15 people. Thats the point that you can't see the bug until "hudge" amount of people comes. I tested with my server statistics, i can see how much CPU working at the right moment when i'am clicking on layered navigation checkboxes. So if more people uses layered navigation it just uses FULL proccesor and presta stops loading pages.
I understand that it's verry hard for you guys to see the exat problem, but the problem is there lying in silence
I had suggestion to use memcache. Could this solve the issue?
Can you suggest any other modules for similar layered navigation?