Router continually needs to be recycled

We have an ISDN line, a router, a VPN Sonic Wall and have been experiencing continued problems with the router. For no apparent reason, the connection to the Internet breaks. I need to recycle the router to the connection started again. This happens usually at least once a day now. We have changed routers twice, have had the telephone company out to check the line and it still is happening. Our ISP says it is not him.

We have a support consulting company working on it but they seems to be frustrated and confused as well. Any ideas?