I apologize you are having so many issues with that device. At this point there are no known issues that the manufacturer has communicated to T-Mobile. I would recommend contacting Samsung directly.
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I walked through some troubleshooting steps with Samsung. After a couple of tries, they suggested reseting the phone to factory defaults. This loses and information stored on the phone so be warned make a back up first. You also have resynch any lists, download any apps and set up any accounts you had set up previously. Basically the phone is returned to out of the box condition. I first was walked through a soft reset through the phone software. This did not work (didn't help still randomly shut down). Then I was walked through a hardware reset bascially catching the phone as it powers on. This seems to have done the trick. So far so good. Good luck to you and hope this helps.
I finally figured out this problem on my second exhibit (the first replaced under warranty with the same problem). It was the battery. The battery had swelled, perhaps from overheating. The swelling made it lose contact with the phone randomly and eventually also made it hard to close the cover.
I purchased a new phone battery for about $11 from Amazon. The phone works perfectly again now.