vauxhall omega 2001 alarm going off?

alarm siren goes off(can be horn or siren)and cannot be switched off until it switches off itself,complete pain.Fitted new siren which was ok for 2 years and now its doing it again.Have checked all contact switches and connectors and they are ok.Also the alarm has a tendency to go off at 5.48am PRECISELY????????????? sometimes!!!!!!!!!!!Anybody got any ideas,thanks.