I have traced this phenomenon to such things as WiFi, Cell/mobile phones and
associated infrastructure.

I got my neighbour to move his WiFi unit from the front of his house to the back
room and my alarm was again stable.  Same thing happened at work but they
couldn't move the WiFi transmitter so I move 4 parking places away from it and
again no false alarms.

Another reason could be the bonnet/hood switch as it occasionally gets corroded
- quick clean should help.

Failing battery is also another reason so get that checked and check current
drain in quiescent state (everything off).