Intermittent MEGA lockups - SM5100b?

hi,

I have a controller board based on a MEGA R3 which is locking up randomly after say 5 hours, then it will stay down for a few hours, then recover and work for a few hours, etc. I have a number of these controllers I've made and the only difference in this one is it has a Sparkfun SM5100B board which is obviously under suspicion. I am aware of the power issues of the SM5100B and have things powered by a separate 12V PSU. However, I notice it (the top of the SM5100B board) gets quite hot and I'm not sure if this is a power or heat issue.

Can someone tell me if the Arduino has a thermal or over current 'fuse' which could cause this behavior?

Note, my sketch is identical on all my controllers and I'm not even talking to the SM5100B board on this one, although it is powered up.

Cheers
Chris