iMac doesn't recognize USB keyboard

This happens periodically when I turn on my new iMac from a shut down, but not always. With a USB keyboard plugged in, I get a message that tells me there's no keyboard connected. It's looking for a Bluetooth keyboard, which I don't have, but no matter what I do with my USB keyboard, the computer can't find it. I've tried different USB ports. They work with other devices, but not the keyboard. I thought maybe the keyboard was shot, but it does work about half the time I do this, and it seems to work fine with other computers.

Any thoughts?