Cisco WAN :: 851 Routers Are Frozen Almost Once Or Twice A Day

We have one cisco 871 and five cisco 851 routers , all those routers are connected to each other using site to site VPN tunnels, so each router have 5 tunnels, we are usong thhese routers for IP phones, each location have 2-3 IP phones and 2-3 computers, all routers are connected to internet using ADSL modems,
 
out of these 6 locations 2 cisco 851 routers are getting frozen once or twice a day,  and we need to reboot them, I already changed the router at one location with new one but problem is still there, but if i look for the logging ( Show logging ) its shows only interface up and down status that because of reboot, i cant find what is the reason these cisco 851 routers have 4 port LAN swithc in built, that still works even router is frozen, i was not able to ping or  access the router not even using console.