Optima xm alarm problem?

I have an old wired optima Xm alarm system with 5 pir, it worked okay until a few weeks ago. It went off itself when in day mode. I replaced the battery and seemed okay for a couple of weeks. It went off again itself. I did an Alarm test and all tests were okay. Last night early morning it went again went off itself twice. The internal sounder and outside strobe activated in all cases. I put in the alarm code and reset okay every time. When I did a memory test it indicates a tamper fault?