Apache2 random 403 error & info server busy logs on Ubuntu

I have a strange situation with apache2. Meanless, random 403 errors. Any page (html, php etc.) normally working. but if i request repeatedly by pressing refresh button of browser. it interrupts & sends a 403 randomly. after a few seconds it works again.
in the error log, i see client denied by server configuration.
main error log of apache says
[info] server seems busy, (you may need to increase StartServers, or Min/MaxSpareServers), spawning 8 children, there are 99 idle, and 137 total children

my current values
IfModule mpm_prefork_module
StartServers 120
MinSpareServers 100
MaxSpareServers 200
MaxClients 256
MaxRequestsPerChild 500
/IfModule

i've increased 10 by 10. from 20. but nothing solved. i've disabled KeepAlive.

What may cause this problem ?
thank you in advance.

a fresh install Ubuntu server x86 8.04.4
Virtualmin from it's website (not from debian repositories).
Linux 2.6.24-27-server #1 SMP i686 – Apache 2.2.8 Mpm prefork
Virtualmin version 3.78.gpl GPL
PHP Version 5.2.4-2ubuntu5.10
Loaded modules:
core_module (static) log_config_module (static) logio_module (static) mpm_prefork_module (static) http_module (static) so_module (static) actions_module shared) alias_module (shared) auth_basic_module (shared) auth_digest_module (shared) uthn_file_module (shared) authz_default_module (shared) authz_groupfile_module (shared) uthz_host_module (shared) authz_user_module (shared) autoindex_module (shared) ache_module shared) cgi_module (shared) deflate_module (shared) dir_module (shared) env_module (shared) expires_module (shared) fcgid_module (shared) file_cache_module (shared) eaders_module (shared) mime_module (shared) mime_magic_module (shared) evasive20_module shared) negotiation_module (shared) php5_module (shared) rewrite_module (shared) etenvif_module (shared) ssl_module (shared) status_module (shared)
Syntax OK