It sounds to me like your battery isn't holding a charge well and is gradually running down. It seems a little early if it's the 2005 you're talking about, but it sounds just like my experience last year. I charged the battery up and everything was fine for a while and then it got weaker. I still haven't got around to installing a new battery but I now keep an emergency starter in the car and I have a solar panel to keep the battery charged up. It was -10 centigrade last night and the car was covered in ice under 4 " of snow, there was still enough juice to start it. I know, I'm living on borrowed time.