Does the OnPeriodicTimer() DOpus.SetTimer() have some sort of time out whereby it switches off after about a few hours ?
Running some tests and left the slidwshow on, at 1 minute intervals, with logging on and for some reason it stopped logging any OnPeriodicTimer() events after a few hours running.
Will run some more tests.
It might not call it again if the previous call still hasn’t returned yet.
Stopped again after about 22 hours this time, the script is still running because clicking the start/stop command button displays the slideshow has ended log.
It just has not been recieving any OnPeriodicTimer() calls after it stops (a log is displayed every time OnPeriodicTimer() is called).
There is a nice list of logs every 1 minute until the time it stopped.
I have not been using Background Changer at all during the testing period and the computer has not been turned off. Using Directory Opus normally, downloading and renaming some images, copying images from one place to another, creating new folders, switching tabs, etc..
Canot think of how to debug if it is the fault of the script, which is pretty simple - display list of images repeatedly at specified interval.
Thinking it is probably either Microsoft or more likely Directory Opus doing something that is preventing the OnPeriodicTimer() handler recieving any events. ?????
Would the machine have gone to sleep during that time?
Sleep is off 
Monitor off is 5 minutes.
Running some more tests now.