squid proxy issue

[Tuan Nguyen <tuan%precedence.co.uk@localhost>]

Hi all,

We're experiencing this strange issue with squid on NetBSD (5.1 RC3). 
During busy hours we're seeing repeated error messages in squid's 
cache.log:

Failed to select source for 'http://...'
TCP connection to x.x.x.x/3128 failed
TCP connection to x.x.x.x/3128 failed
TCP connection to x.x.x.x/3128 failed
...

x.x.x.x is the upstream proxy as configured in squid.conf: cache_peer 
x.x.x.x parent 3128 0 no-query default name=x.x.x.x:3129

As a result, on client machine a user would see "Unable to forward this 
request at this time" message on their browser. This doesn't occur when 
network is less busy and we're seeing the same issue with different 
upstream proxies. Is there anything we could tweak on squid/NetBSD to 
overcome this or parent proxies are the one to blame here? Any advice 
would be great. Thanks.

Regards,
Tuan