"failed to connect to upstream server" - HELP

Does anyone knows what this error means "failed to connect to upstream server" i am getting this message on browser, after i hit the F5 button the page is loading again, this bug occurs in higher frequency when the site's traffic is high.

i suspect that is the connection between apache and tomcat ( using proxypass).

how should i resolve it ?

p.s. i have tried to increase the ThreadsPerChild from 250 to 500:

<IfModule mpm_winnt_module>
ThreadsPerChild 500
MaxRequestsPerChild 0
#Win32DisableAcceptEx
</IfModule>

I think it helped a bit..

Thanks in advanced.

Ofir Ashery.