Sorry, this will be too late for you, but gonna leave this here in case someone else searches for it.
Fixed this problem tonight. Apparently, tightening up the phone fixes this (the power button unit does not attach to the rest of the phone, it merely comes into contact with the board through proximity). You can tighten the black screws on the motherboard (hard) or press hard on the upper left hand corner when taking the back cover off where the battery and sim are (easy - no real disassembly required).
For a REAL and PERMANENT fix, you can totally disassemble everything and pry the tiny metal prongs in the power button area to a higher angle, ensuring that they connect effortlessly. This is considerably more difficult, but will prevent the problem from returning- FOR GOOD.