Does this happen for every URL or for just a few? Which timeout value are you using right now? What's the exact name of the timeout setting you are using?
You could do a tcpdump on the MWG box that shows the connection to the webserver which times out.
Edit: You might also want to look here: https://community.mcafee.com/message/234571#234571
the Initial connection timeout specifies the time that the Client has to send a request after the connection to the proxy. That's not relevant for this case here, as it's the server connection that runs into a timeout.
The value you need is the normal Connection Timeout, which is set per default to 120 seconds. That's already a lot and I don't believe that the Google webserver doesn't answer in this timeframe.
To examine this further we would probably need the connection traces, to see what exactly is going on.