Phone shutting down

Where to begin....so I've had my S3 for almost 2 years now and it has run smooth since I bought it. However, about a month ago, my phone just shut off and did not want to turn back on. I tried taking out the battery, putting it back in, using a different battery, connecting it to a charger, but still, nothing. When I would try turning it on, the SAMSUNG logo would flash, the splash screen would play, but then it would just shut off before fully booting up. I tried going into recovery, and I would be in recovery for about 10 seconds before it would shut off. I just tried turning it on for the next hour, and eventually, it turned back on, and stayed on. I thought it was a ROM problem (Was running MOAR v.8) so then I flashed PacMan ROM and my phone was fine. I would reboot to recovery and everything ran great. But....the same problem occured again about two weeks ago. I did the same things as the first time, and it eventually turned back on after about an hour and a half. I change ROMs again and now I'm on SlimKat and things were smooth.....but...once again.....it shut off yesterday. I tried the same things as the previous times, but it would not turn on. My phone would reach the endin of the SlimKat splash screen, but shut off. It has been almost 24 hours since this happened and it still doesnt turn on. Now I am getting worried because it has never stayed off for this long.

Something that may have caused this is my headphone jack replacement. I replaced the headphone jack module myself about 3 months ago, but It was a simple replacement, so I don't think this could be the culprit. I am just lost now and don't know what my problem could be. I have no insurance on my phone, and I am waiting until the LG G3 is released for sprint so that I can upgrade, so I really need this phone to work for another month.

Does anyone know what my problem can be? Any ideas?

Edit: My phone was on the SlimKat splash screen for about two minutes before shutting down. Felt like adding this, maybe it'll help.

Could this be a soft brick?

roirraW "edor" ehT said:

I really doubt it's a brick of any kind. More than likely your power button is going out. It's very common. You might get away with just taking the phone apart and really cleaning around both the button that you push on from the outside and the actual switch on the inside.

Mind you, this is what I've read and some people have fixed it doing this, but I haven't had this problem...knock on wood.

Sent from my Sprint Samsung Galaxy SIII on TouchWiz 4.4.2 with Wicked X v6.5, Ktoonsez' latest kernel and the latest Philz Touch Recovery.

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ThatGuy94 said:

The power button is probably going out. My S3 which I had for about a year and a half just completely stopped turning on and would only make it as far as the initial Samsung screen when you first press the power button before it would turn off. This problem can't be fixed due to the actual pad is a part of the motherboard. Since Sprint couldn't tell that my phone was rooted because they couldn't turn it on either, the only thing I could do was trade it in for another S3.

Also, when you put the battery in the phone, does it automatically turn itself on without pressing the power button? If so, this is definitely your problem

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Thank you both. I will try to clean out the power button and search for a tutorial. Yeah, it does automatically turn on. Does this mean it can't be fixed?
Edit: Fixed it by watching a video that said to just bang the power button side on the table.....Thank you for the help!