Yes it is a known issue for some.  Rich Renken - Product Manager for the M13 joined in a lengthy thread on this here: http://line6.com/community/thread/2929?start=0&tstart=0 .  It seems to happen once and then never again for most.  I know that Rich has had it happen to him at least once and that it was under investigation.  I had it happen once within the first few weeks of having the M13 last October, but assumed a faulty mains socket at the time - which it still could have been.  I don't know whether a common cause has been found yet or not.  Check the rocker switch at the back of the unit as apart from the internal relays, this is the only mechanical component that could turn the M13 off and on - otherwise check the PSU isn't running too hot (they do get a bit warm) as it could possibly be a thermal safety cutout I suppose.  It does seem to be very much a random thing and as such, very hard to pin down.

Nick

Well i have had mine for 11 months also and it kicked out on me in the middle of a gig tonight! I rebooted and re plugged everything and it kept shutting off! So i had to run it through only my CryBaby Slash wah and it sounded terrible because (due to space issues) i had to run without an amp. So i can'thave this happen to me, I play every Friday, Saturday, Sunday, and Monday. The lights would flicker and go crazy and just kick off and then it would turn itself back on and then shut off again. Please advise.