... and now it works with a lot of false alarms....very unreliable this x-10 stuff
The false alarms could be caused by a problem with any of the components in your setup.
It is generally recommended to avoid HouseCode "A", and unit code A1 specifically. This is the factor-default setting for all X10 devices, and you may get stray commands from neighbors whose sensors have low batteries, or are not set up correctly.
However, I would suggest you check the motion sensor first. In my opinion, your "false alarms" may be caused by the motion sensor being triggered by animals, leaves, or other motion (wind blowing the nearby trees or bushes, even), and ever time the chime is sounding, it is because the motion sensor is triggered, not because of other stray signals. Repositioning the motion sensor may help, if that is indeed the cause.