Troubleshooting (Score 1)

I have a buddy who likes to say that there are only 3 steps to troubleshooting -

1. Is it plugged in?
2. Is it turned on?
3. Is it configured correctly?

Through various mis-adventures, we've had to append 'correctly' to both steps 1. and 2.

Seems possible that 3. might be a likely culprit here - but, I heard no mention of the newness of the problem. So a new setup (for example) may likely not have been plugged in correctly.

Also I've had to add a Step 4. or at least a step 3b. - Is it new code?

The whole event sounds like a Spanning Tree loop - L2 broadcasts/multicasts just being forwarded endlessly and symptoms identical to what was reported. I have seen both code bugs and mis-understanding of new code defaults lead to such a thing.