Sounds like you have either a wiring, or outside sounder tamper issue,
check the wiring for your outside sounder, and also the tamper switches, front and back on the sounder itself.
Ensure that there's no corrosion to these tamper switches.There's defo a intermittent tamper fault somewhere.
You could link out the tampers,but that should only be carried out by anyone who is aware of the dangers, and understands how the system operates electronically.
Sorry Rodge, but that's all I can suggest, other than getting someone in who can trace and correct the tamper fault.
Maybe the guys can offer more advice,and also the control panels backup battery may need to be replaced.All the best with it,and good luck,whatever you decide.
e manno
May 2012