poor uptime with varnish

Hi, 
On my vds server i installed cwp and activated nginx apache php-fpm and it seems to work fine, i use uptimemonitor.com to check uptime.
I activate varnish for better performance nginx-> varnish-> apache-> php-fpm and uptimemonitor.com reports 2 or 3 downstates for one day. I tried to monitor the ping but even during the downstate the ping was successful.
Does anyone have the same problem or can you advise me what I should do to investigate and fix it?

use disk cache it is more stable then malloc