Optima G3 Compact Alarm System?

Hi,

On this alarm, I have the Chime set to go off, for my front door, back door and garage - all of which are on the same single input into the alarm circuit board.
It has recently started chiming once (as needed) while the front door is open. When I open the back door or garage door, then the chime goes repeatedly ever 5 seconds until I close the door (it never used to do this). Has anyone got any ideas of how to solve this please. Thanks.