No, it's a phpbb forum, so it's very small tasks.
For exemple, a member want to read a reply in the forum : error 503. He can't access to this forum without clear his forum cookies.
I have 50 000 uniques visitors per day, but only a little part got this error. One day, this member receive this error, an other day it's an other...
The error come in rush hour(load average=3) and during the nigh(load average = .02)