a mate of mine had the same sort of thing happen to his xr2, i spent hours on it to find out what was wrong, everytime i jump start it it was fine but when he would leave it over night the battery was dead by morning, we even fitted a new alternator and battery but still had the same problem. as we couldnt solve it he had taken it to a ford dealer and thay said thay have come accross it before and it was the starter motor draining the power. try another starter motor