I ad the same problem on a astra gte,it was a drainage problem,A friend tested it on a meter and i was losing volts,i then took it to an auto electrics who charged about �20 plus parts for a full test.They found a broken wire and as no parts were needed thats all it cost.As you can tell i am no mechanic, its just advice based on my experiance.hope this helps.By the way, that was a few years ago so the price will have gone up abit.also you might av an interior light stuck on, check the boot light,glove box(if you av one)