Keyboard Shortcut for Sleep not working

I've recently tried using the keyboard shortcut for sleep, option-command-eject. When I do this the computer goes very briefly into sleep mode with the slowly pulsing light on the power button, but then, almost immediately, becomes active again. Any application in use or opened thereafter becomes unresponsive and has to be Force Quit. It seems something in the sleep activation gets interrupted and stalls.

*Sleeping, using the ordinary menu or dialogue box commands (including from the power button), works OK.* Going through a full sleep/wake cycle, using one of these methods, after this happens, seems to fix any lingering problems from this stalling.

Since I thought this might be an issue unique to this G3, I posted this question on the iMac CRT forum. (I see now this is likely a Tiger question). From there I did some troubleshooting, which wasn't able to resolve the problem, including zapping the PRAM (the PRAM battery is less than a year old and computer has only been off wall power for a little more than one week since changing, including time off during electrical storms over the summer).

The option key itself is functioning OK. Sleep and shutdown with the dialog box from control-eject works OK. I can shutdown using the Control-Option-Command-Eject keyboard shortcut.

I've tried unplugging any peripherals. I've repaired permissions and verified the disc.

I'd like to be able to use this shortcut and to understand why this may be happening. I know it would be easy enough to sleep quickly from the power button, but since this is an older computer, I wouldn't like to overuse that switch.

Can anyone suggest what else I might try? Thanks.