MiFi: 691 error, then slow speed, then disconnect

drwhom

After months of flawless service, my MiFi 2200 is now behaving strangely.  When I try to connect, I keep getting authentication (691) errors.  When I do get through, I get slow speed for a short while, and then the device disconnects within a few minutes.  In case it's an issue at the tower, I'm in ZIP code 20037 (West End, Washington, D.C.).  Any help or suggestions will be appreciated.  Thanks.

drwhom

Then again, it's not clear to me that it's the tower, since my TP2 continues to work perfectly at the same location.