After going through three 8700's in six months, and still having the same problem, it's now solved...easy fix. First I thought it was the battery...WORNG...
I went to a T-Mobile store and changed to a new sim card. Problem solved.
It's been four days and so far so good. Prior to this the phone would re-start about 15 times a day.

My guess is that becuase I have had my sim card in and out of many phones of the years, it just needed to be replaced.

Hope this helps those with the same problem.