There could be so many reasons, but frankly only one answer - time for a new one.
There are too many variables and too few diagnostic indicators on a system of this age.
One (or none, or more) transmitters could be faulty and causing this, but the system is only 1 way and has no proper interrogation mode.
It could also just be the tamper on the outside bellbox, but I would expect this to happen if it was set or not.
To check a transmitter at 'base level' or to monitor possible background interference requires an operative ideally equipped with a long obsolete
Scantronic Reciever tuner/tester box.
It really is time to upgrade this system to a more modern type of wireless alarm (or, wired if you can).
Why put up with an ancient, unreliable alarm, comprised of obsolete parts, to look after the most expensive asset you own?