fangs

My mom drives a 2004 chevy caprice 3.6L V6 and two days ago while returning back home she said the car started funny. On the first try the engine did not start, the second try the engine started but not with the same "power" it usually has then it switched off.

I checked the car and on the first try the car switched on, full power. I switched on the interior lights and all worked and were bright, also the headlights were working fine and seemed to be bright as usual (it was daylight). Radio also turned on fine.

I'm not experienced at all when it comes to cars but I popped the hood to have a quick overall look and see if there are any leaks or maybe spot something that looks out of the norm. It looked fine to me and I even inspected the car battery since it's been 2 years since I replaced it. The battery (AC Delco) has a green ok/black need to replace sort of window and it's still green. We drove the car and even had the A/C on without any problems at all.

It's been two days since the incident, and yesterday (after it has been parked and not driven for over 24hrs) we tried to turn on the car but it didn't work. While today morning it turned on and worked fine.

Any ideas what is the problem?