Originally Posted by brain boy
it shuts down completely(i need a long press to turn it back on)
sometimes it stays for 5 minutes sometimes 20 minutes
and the battery is fully charged
OK, it sounds like you have the poor battery contacts problem. To be sure, you can do a hard reset to rule out any software issues. If you have no backup program installed then you will lose anything you have installed to the phone however.
So you could go straight to a simple fix for this problem. That is a strip of card to push the battery more firmly against the gold connectors. First make sure the connectors are all clean:
In some rarer cases the connector block with all the gold contacts has actually become partially detached from the board - perhaps a hairline crack in the soldered contacts. Hopefully yours does not have this problem though.