Thanks for the link. I tried what they suggested, but still no luck. It seemed to be aimed more towards solving issues on the iPhone side, and I suspect that my problems are on the Mac side.
Before I started using the iPhone, I used to use a Novatel 3G/CDMA USB modem for my away-from-home internet connections. I'm wondering whether the Novatel installation may have deleted or inactivated some other USB drivers. The Novatel option still shows up in my network preferences, and it is the only one that uses USB. I deleted it from the preferences and I went to the Novatel site and followed their instructions for removing the Novatel software, but it didn't help.
I'm planning to upgrade to Yosemite in the next couple of weeks. So, I'll wait and see if the problem persists after the upgrade.
