Could the time delay in being able to reproduce relate to some piece of code that has a timeout (thinking login timeout, cookie expiration, auth timeout, that sort of thing.) Or likewise, if the computer in question has multiple shutdown phases, like how many computers today "sleep" to RAM, and then an hour later sleep to disk in a more hibernatey fashion and fully power off? (Or some weirdness like how Windows shutdown now is ostensibly a hibernate, but a reboot is actually a full "power down power up" without shutting off power.)

I like @[email protected] 's take on being wall-clock-based. I once had a bug with some software that would just go belly-up on certain days for no reason whatsoever in a datacenter 2000 miles away. After having worked on some bare metal servers in the past and learned all about thermal issues firsthand, I checked the weather in that region. It only seemed to happen on extremely hot summer days, at the day's temperature peak. Turns out the datacenter vendor had a cooling problem in that section of the DC and they were unaware of it...

Crazy sometimes how bugs manifest.