Most likely a loose connection now... try tracing all the wiring from the battery inwards to see if anything may need to be touched up with a dab of solder. I had an old Samsung that used to get dropped a lot and the battery never used to fit back in properly where it was always contacting the leads inside the phone.
Failing to find an obvious cause in the power wiring means to expand the search... examine the power-on button to see if it's been crushed a bit and is sporadic now... also the ribbon that comes off the keypad may have pulled loose a bit from it's connector... things like this. If you can't find anything glaringly obvious with the wiring, examine the chips inside with a light that goes across them to see if there are signs of overheating... shiny black spots in the middle of a normally matte-black chip, etc. If it's something at the component level like that you'll just be better off to get another one, doing soldering on this stuff is like micro-mini brain surgery.