Samsung Captivate :: Keeps Turning Off Itself

I've been trying to see if i have a defective unit in that the display is taking up more battery than standby when the phone is not in use and so far my phone has turned off by itself 3 times in the past 13 hours. I had it 100% charged last night before going to bed so i unplugged it from the wall, put it on the nightstand in airplane mode, and i woke up an hour late this morning because my alarm didn't go off because my captivate turned off at some point in the night. I fired it up and it had 94% battery. So i immediately put it into standby and put it in my pocket and went to work. When i got to work i pressed the standby button and nothing happened, so i held it in and voila, the phone is turning on. So on my way to work it randomly shut down again. So now it has 90% battery so i put it on standby again and let it sit for 3 hours. I just checked on it and it had shut off again. It's now at 88% and shows Display=54%, Cell Standby=15%. I think this thing is completely whacked. Between this and the GPS not working i think i'm done. I'm definitely going to take this one back, but i'm concerned about getting a new one that's even more screwed up as it seems there are such few good working copies of this phone out there. Is there some setting that turns your phone off after a certain amount of standby time that i might have somehow mistakenly turned on? I feel like that would be a ridiculous setting to have, but it seems like there are a lot of options on this phone so ya never know.