Lost Configuration on Cisco 1720 Router

I have multiple 1720 client end routers. On occassion, a router will apparently lose its configuration settings for no reason. I arrive on site, console into the router and am rewarded with a Router> prompt. I have been looking for a pattern to determine the problem and have hit a brick wall. Could a signal from a telco company cause the router to reset? I have two sites that have a really bad problem due to this problem. I have swapped out routers and the replacement routers encounter the same problem within 60 days. One site has been having this problem once a month.