Hi, Peter, Craig ... thank you both for replying!
Nope, Craig... I do not have a cat! Forgot to mention that as being an important piece of information! but thanks anyway.
Peter, I don't think it is happening on a consistent frequency (e.g. every time on certain day/at a certain moment), but I will check once more. I hooked up the system after several years of no power ... the system was totally reset when I hooked it up again and I did not program it since then. Also, the remote control seems to be pretty basic (i am not sure what is meant by two-way-remote).
Any other idea's? I though this might be a common problem .... Thank you. Michiel.