Cisco 851w drops WAN connection

Hello everyone.

I?ve been with this problem for a while. After reseting the router, it starts working fine. After 2 or 3 days it stalls the Wan connection or works at a very low rate loosing packets all the time ( tested with ping from within the router ).

Seems to be the router and not the external connection ( fixed IP with a WiMax connection )as only reseting the router fixes everything.

Has anyone found this same problem and fixed it?
Thanks a lot for the help. This is driving me nuts.

Rgds
Ricardo