Do some ping test to check connectivity on all steps:
a) From your Computer to Endian: On your windows, execute
ping <<YourFirewallIP>> -t. Also execute
ping www.google.com -tb) Connect a monitor to the firewall and from the console execute
ping www.google.com.
Let it running.
If you see any strange in console, this could lead you to a possible solution.
Then, when you have a connection problem check the console output to see if Endian Firewall also loses ping replies.
Do some other tests too. From endian console, use the top to see if something is eating up too much CPU time at random intervals.
Another problem could be some hardware problem, related to incorrect NIC drivers.
How your HTTP proxy is configured?