Alienware X51 R2 occassionally crashing and not starting up with single beeps

Hi all,

I have an Alienware X51 R2 with an i7 3770, 16gb RAM, AMD rx480 and 330w PSU. The motherboard has bios A11, which I can't seem to be upgrade from. It's been running fine in the current spec for about a year. Recently, I've been having a problem where the computer will hang after about 5-10 minutes, forcing a hard shut down. An immediate effort to restart usually leads to single beeping (which I believe indicates motherboard failure). If I then unplug everything and leave it for a little while, plug everything back in and turn on, it normally works fine until the next hang and hard 

shutdown.

Note that the computer will hang even when idling on the desktop, under essentially no load. 

I've tried resetting the CMOS, but the problem persisted after this. I can't tell if the potential problem is the PSU or motherboard. I would have guessed the motherboard, but do they often die slowly like this, or could it be corrupted BIOS?

Thanks for any help/suggestions