How do I prevent my Optima Plus alarm system going off!?

Twice in the past week my Optima Plus Home Alarm system has gone off in the middle of the night & I have been unable to reset from the control panel. I have had to switch off at the mains! When I reconnected the system it opertated fine until it went off again at 04:05 this morning. The activations appear to commence immediately after what I believe to be a cut in power that must only last a fraction of a second as I can hear other alarms in the area around me going off. Will changing the battery, last changed August 2011 remedy the problem. This has never happened previously since the alarm installation in 1993.