Is your hosting provider GoDaddy? We are having a similar issue and had to disable the USPS module. I spent an hour on the phone with them yesterday and am told the issue has been escalated to the network administration staff.
If you're using another provider, then never mind. :)
In any case, what appears to be happening on my site is that the proxy server is taking too long to respond. Thus the USPS module takes a *long* time to retrieve the quotes. Meanwhile, the MySQL server timeout is set for 20 seconds. So, by the time the USPS module completes or times out, the MySQL connection has been closed by the server and the error message is generated.
To confirm/replicate the issue try running a script that only fetches a URL with cURL. Does it take an extraordinarily long time to complete?
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