impreaza 52 plate alarm goes off when it likes.

Goes off 1 morn no reason, cant stop, goes again few hours later so lock it with key so alrm not set but still goes off. lights not flashing on this occassion. take to garage who plug in but find no fault. ok 4 few days then goes again this time with lights flashing, will do sequence(about 10-15 alarms) then stops. ok for while. now it will go off as soon as you open with key fob. have to wait 4 sequence to finish sounding then ok again. now when you unlock/lock it will beep once or twice as if not set right??? even while alarm sounding you can start engine and drive it while going off???!!!!
P'd off now so r neighbours and we r none the wiser. has anyone heard off this or what to do???? getting desperate.