How can I fix my Yale wireless alarm system?

I have a Yale 3000 wireless alarm system that has worked well for several years. Recently, I have started o suffer a spate of false alarms (very annoying for neighbours when I am away from the house and at night)
I have cleaned all of the PIRs, the siren box and the keypad and replaced all of the batteries.
Most recently, no faults occurred for a two week period but the problem has now recommenced.
Could this be a tamper control fault on the siren box caused by expansu=ion/contraction because of the hot weather. Odd if that is a suggestion because the box is mounted on a wall that does not face the afternoon sun and I would have exoected all of the movement to have subsided by nighjt-time.
The false alarm does not commence on setting the system and can occur after several hours of after just half an hour or so.

Brian
August 2006