if it is fine to begin with, and then it goes after a little while, i would say it is not a hardware problem, but a software one.
especially as you have tried it on more than one ROM, it suggests that you may have installed something else on to the device which may be influencing it.
Hard reset and sycn your contacts etc back to the device, but do not install anything else, or do any tweaks at all.
let it run out of the box without any changes for a 24 hour period.
does it still happen?
if yes, send it back.
if no, start putting everything back on one by one until the problem re-occurs, trying the device for a couple of hours after each install. when it fails again, you have your suspect which is the last thing you installed or did before it failed again.
course, it could be a dodgy device, but make sure before you go through the hassle of sending it back, cos it could just be some software that doesn't like wm6.1.