Why is my SCH-S738C phone turn off by itself when battery is good?

SOURCE: Samsung SCH-340

t happened to me also exacatly as you described. I went to Alltel and they did some tests & replaced the phone. I had insuramce on it. They gave me a loaner & I sent it back to the service center & they replaced it without any additional charge,

SOURCE: Samsung SCH-u340 Cellular Charging Problem

i have the same problem but i belive i broke the hole to the charger by constanly shoving it in and shoving it out might be the same with you

SOURCE: Samsung sch-u550 cell phone shuts down by itself in stand by mode

bad battery, go and get replacement

SOURCE: My Samsung SCH-R450 keeps calling people

its a technical problem u should contact a repaire ur problems can be solved at low cost

SOURCE: phone keeps turning off and on by itself

hello try restoring the phone to the default factory settings
if this doesnt work, get a new battery