Had this sort of similar problem some years back with my wifes Astra. Opel were really baffled as well and spent some time with a donor car swooping our parts etc and we finally found out that it was linked to the wiring on the initial tracker device that was removed by previous owner. So it was electrical related or earthing. Depends were u take it, if its a dealer they will do a text book approach and identify the control unit etc. to be replaced before doing any fault finding like a old school electrician.
I would start with some basic fault finding, did u do any work on the car etc and start there before checking bigger items and replacements.
Good Luck