I have received word that Apple has confirmed that a subset of Macintosh Performa 6360/160 and Power Macintosh 5260/120 and 5400/120 computers equipped with 1.2GB hard drives "may experience intermittent flashing questions marks during a cold boot." The cause appears to be a problems with the hard drive itself. The drive problem has no other effect and will not cause a loss of data on the drive. Rebooting is the simple work-around, as the computer will usually start up okay from a warm boot. Apple will replace drives having these symptoms.
This appears to be different from the blinking question mark problem previously reported here - as that problem occurred only on warm restarts.