ef/door contact problems on scantronic 9800?

Hello can anyone advise please - I have a scantronic 9800 alarm system and it developed a fault just before Christmas it is showing ef which hubby knows is a door contact problem -he found a loose wire in the main contact and sorted that out plus he packed it out so that it aligned better with the magnet on the other side of the door and after testing 6 times (just setting and waiting to see if the alarm goes off)all seemed OK and it worked for several weeks with no further problem
The problem is that is works 90% of the time but sometimes after setting it the alarm goes off for no reason that he can find and shows ef again and he can find no other faults and when testing it seems OK but then reverts back to the alarm going off after setting it now and again - it is unpredictable
Any ideas please
Will a new contact sort this out and if so where is the best place to get one
Thank you for any help/advice you are able to offer