Air Card connection (by 2 users)

I am using five D-Link cams 6620, all with Kyocera routers and Verizon air cards. Two of the five cams have been operating fine since May 2007. Three of them will lose their connection... sometimes after a few hours, some after a few days. Checking with my cell phone (same service provider), I can register 4 or 5 bars of signal strenth at the cam site. All the sites are in tall buildings.

Rebooting, moving the antenna, turning the router closer to the window... none of that helps. Thank you in advance for your ideas and help.