> I don't know. It would be difficult:
> - in happens only once or twice per day
> - the timers depend on my personal environment: checking email,
>   news, Bitcoin value, the todo-list of my org-file and so on
> - some timers depend on idle-time

> Would it help, if I a add some code to the start and to the end of
> each timer function, that prints interesting information to the
> *Messages* buffer? And when it happens again, we would get some
> trace?

After the timers stop, do they start up again when you 'kill -s ALRM'
the Emacs process?

Maybe you could put a print statement in the handle_alarm_signal
function to confirm you get the signal. If you do, then perhaps start
debugging at the do_pending_atimers function.

If the problem is that you're not getting the signal, write a small
program that implements a simple timer using SIGALRM, run it
continuously, and see if it stops too under the same conditions.