306 Ultrasonic alarm sensors: How to fix ?

I have a '97 (new model) 306 GLX.

The alarm has been randomly going off recently (sometimes within 5
minutes of arming, sometimes hours). I have followed the instructions
for de-arming the US sensors (found on this group - thanks) and when I
do that, the alarm never goes off, so I'm fairly sure its the US
sensors. I've searched this newsgroup, but I can't find answers to the
questions below - sorry if I missed them.

Problem is, I'd like to have them armed the whole time, so:

Anyone know how to fix them ?

Can anyone give an account of how they work ?

Is it possible to trim their sensitivity ?

Can they be triggered by slow continuous changes in air pressure, for
example, when the car is in direct sun light, or a very cold evening ?

Does it make any difference if their is lots of stuff in the car ?

FYI - the problem first happened on a cooling summer's evening with
bikes inside the car, and parked next to a Fiat with known alarm
problems.

thanks

tom saffell