Losing modem connection with MBR900

I have a Sierra U301 modem in a MBR 900 router with Sprint Wimax (4G) service. Periodically the connection between the modem and router loses connection. My modem still has a connection light on it but the connection light on the router turns off. If I reboot the router, either by powering off or through the software or if I unplug the modem from the usb port and plug it back in I regain connection. There is no rhyme or reason when it does this. I am on my second router and both have had this issue. I have contacted Cradlepoint and they said to do an upgrade on the firmware for the router and modem, which I had been doing and did again with no improvement. Any thoughts on what might be causing this issue?

Thanks

Hi,
Have you ever used the U301 without the router and did it used to do this? How good is your signal strength especially when you lose connection?