How can I diagnose a bridging (ethernet) loop?

You will probably have MAC addresses flapping between ports.. look for MAC_MOVE_NOTIFICATION (or similar) errors in:
sh logg

Now to find the port:
sh int g0/1 controller
look for out of ordinary Multicast and Broadcast numbers. Any collisions are a bad sign.

Last but not least, you can't log in, because the CPU is pwned :)
sh proc cpu
How is the switch doing here? If it's an L2 switch only, you don't want anything above ~10%



Enable storm control on the ports to limit broadcast

Distribution-Sw1(config)#int  range f0/21 - 24
Distribution-Sw1(config-if)#storm-control multicast level  5
Distribution-Sw1(config-if)#storm-control broadcast level  5

Drop broadcast o multicast level if they are more than 5% of the bandwidth

An alternative action could be  to shutdown the ports:
storm-control action shutdown