Sometimes my guest always Kick Out

[geevest.com]

Hello i have a trouble with my site.
i dont know why.
sometimes if all guests online in my forums.
example : Currently active users 1000(400 members and 600guests).
and then several minutes later all my guests always kickout from my site :
example : Currently active users 400(400 members and 0guests).

what's wrong with this?
and im always get a blank page for 1minute.its always happen.

i take VPS server.if i get a blank page and then restart HTTP Server (Apache) Its Normal again.
thanks

-bash-3.1# w

Philip S: 14:58:44 up 9 days, 1:23, 0 users, load average: 22.77, 24.78, 17.25

Philip S: USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT

Philip S: this is the current load on the vps

Philip S: 8128 nobody 16 0 37580 21m 3724 R 36 4.2 0:00.61 httpd

Philip S: 7897 nobody 15 0 34380 17m 3724 R 15 3.5 0:00.51 httpd

Philip S: 9809 nobody 15 0 33752 17m 3604 R 15 3.4 0:00.08 httpd

Philip S: 7955 nobody 15 0 36412 20m 3700 R 13 4.0 0:00.30 httpd

Philip S: 9315 nobody 16 0 32268 15m 3412 R 11 3.0 0:00.09 httpd