You may have a bad sensor setting off the alarm (I have NO idea where the sensor(s) are.)

But first, I"d look at the battery. How old is it? How do the battery cable terminals look? You want a reasonably new battery and clean connections between the battery and cables.
If yours is a white Varta battery, it's original. Replace it now, today. THEN see what you end up with. As doc says, these cars are VERY voltage sensitive.