I'm not sure if I'm alone with the problems I've been seeing in my Gmail chat lately (Google Talk built into Gmail). I'm using Firefox and I usually keep my web browser open all day. In the last couple months, I periodically see the error message, "We're experiencing technical difficulties that may prevent your chats from being sent." And none of my contacts in Google Talk (within Gmail) are displayed. When I refresh the page, Gmail won't load at all. I usually end up having to clear my cache, restart Firefox, wait a few minutes, and try again, but even then it still won't work. It usually returns after a few minutes. I deleted my local Firefox user profile and recreated it and also disabled all of my Firefox add-ons but nothing seems to help it.
One possible explanation for this weirdness is that the network I'm on uses content filtering (SurfControl) which could be heuristically scanning for suspicious network traffic (such as the periodic "pings" within my Gmail chat). I am using the encrypted Gmail site. The network content filtering could be somehow choking or inhibiting the traffic to Google's servers which may be causing the service to mess up. If you have any suggestions, leave them in the comments.
Update: Later this afternoon, I received another error that said, "Your internet connection is experiencing problems or your network administrator has blocked Gmail chat. Learn more" A few minutes later, I switched back to my Gmail and everything was fine. I've sent in a trouble ticket to Google and I will post if I find out more.