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.