I am having a read through your posts and a bit of googling. Why some don't and some do is a hard one to explain. Almost as bad as intermittent faults that always fill me with dread when they arrive.
Is it a dns fault where you can ping the ip but not the FQDN.
Also are clients all the same? win version / os ?
I know clutching at straws
have a go at pinging some ip and have a look at the network settings maybe some where dns / default gateway ...
I would go back to basics and turn off the proxy.
Check all the clients.
Proxy on, no filters transparent.
Check all the clients.
Bring in your filters.
Check all the clients.
Will have a closer look at your post. If you find any oddities please post away.
If you need to chill that RC1 of 3.0 is ready for a virtual machine test