Cisco 2511 cpu usage

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Hi there, I have 2 modems for dialing in connected to a 2511 router with some rj45 to 25 pins serial connectors. Somehow router shows 100% usage and when modems turned off is almost 0%. How can I find out what is using the routers cpu? It could only be modem DIP switches or rj45 to serial connectors since routers worked OK before we moved to a new location and we might have changed just the phone cable going into the modem and connectors. Router configuration has not changed, neither has the os(12.1 2b). Thanks, Calin