I would be gueesing a bad earth causing intermittient issues. When this kind of fault occurs suddenly it's often very usefull to think what has changed recently and that in your case is the alarm. You need to go over what the alarm people did with fine detail. Even if they removed it you need to hassle the guy who did it and check step by step what he did. Any joins they made or cut into should be checked, earths in particular.

Well my 2 cents worth.... good luck