I had that problem a few months ago with a Rover 216 SLi. Have you found the battery energy is also being drained? My alarm would go off in the middle of the night at horrible hours and sometimes the car wouldn't start in the morning so I had to find someone to jumpstart it every time.
The problem was the battery was obviously being emptied and everyone told me the battery was the cause it was actually the alternator that was drawing too much energy from it when it was turned off and it did it so irregularly that twice I took it to the garage and they said there was absolutely nothing wrong with the car. When I changed the laternator the problem went away!
Check how much energy your battery loses, you might just need to replace it.
Di, July 2008
