I have 3 P2100 switches each running off a separate port on a 2862 router. Two show on average a CPU load of approx 10-20%, but the third is consistently showing anything between 45% and 74%. There is nothing unusual running out of the ports (a mix of AP903s, and ports serving various wired data links to PCs etc. I have tried shutting down each of the ports in turn to see if one of the attached devices is making a high demand on the CPU, but there is no discernible difference as each port is disabled and then re-enabled. I've checked data throughput, but no obvious source, and nothing seems to be making heavy demands on the LAN. I've also checked for loops and storms, but no sign (and almost none of the attached devices is likely to start a storm). Switch otherwise seems to be running happily, so maybe it doesn't matter? Any suggestions welcomed.

Problem solved. Turned out to be a PoE device which, when not in use, was generating significant CPU usage to keep itself awake. Totally counterintuitive to track down.