How can i fix my Accenta8 mini alarm, it works intermittently?

my Accenta 8 mini alarm works occasionally, and sometimes stops working for a few days at a time. The Reset function does not always work.

I've asked a local alarm technician to have a look and he managed to reset the alarm once using the key panel, but he said he did not know the system and was unable to fix it.

When the alarm does work the tamper light on the keypad usually stays on (when the alarm is deactivated), but at least the alarm can be activated and is triggered by the infrared sensors around the house.

Occasionly the when I try to arm the alarm by entering the code into the keypad it gets stuck on a repeated ring or warning noise on the keypad after I entered the code. the alarm does not activate and I then have to re-enter the code to disarm the alarm (and stop the noise). This has happened a few times and the alarm can usually be reset and then re-activated, but the last 2 times the alarm would not re-activate after its got stuck. It could only be turned off. I've had to wait weeks and try again before the alarm would start working again - as if resets itself after a while?

* is this an old system?
* if it can't be fixed can I replace the main alarm unit (reusing the PIR sensors and keypanel already in place)?