Sounds like intermittent problems connecting to USPS ...

Normally, it either works or does not work ... the intermittent usually happens when USPS is having issues ... this is a link to one thing that checks the USPS connection:
http://www.shippingapimonitor.com/history.html?api=usps

You could check with your domain hosting company to see if they are having issues or explain to them that you are having intermittent issues so that they can check for these problems ...

Usually, you will get logs for this problem to help identify times that this has been happening ... they will be in the /logs directory (/cache on older versions of Zen Cart) ... but this issue is either your Domain or USPS ...