Number one cause of random re-booting is heat due to clogged cooling
on CPU's / Graphics Cards / PSU's and this is common to any OS, so
first thing to do is open up the system and check for fluff and dust
clogging the insides.

PS - if you have a graphics card with fan the fan is underneath and
not always easy to spot. One that's often missed

Also, when you have the case open and are looking for a clogged cooling
system, take a look at the electrolytic capacitors on the motherboard
near the CPU. If you see any with bulged tops, that will also cause
problems. They can be replaced by someone with the proper equipment, but
often it is just easier to replace the motherboard.
http://en.wikipedia.org/wiki/Capacitor_plague

I had a machine that would freeze at modest levels of CPU activity and I
suspected overheating, but the insides were clean and all the fans were OK.

Turned out to be a cracked plastic fan bracket on the CPU and it wasn't applying
adequate tension between the CPU and the heatsink. Didn't find it until I took
the heatsink off in desperation and the plastic piece fell off.

Just shows you need to investigate *everything* - once had a system
that defied fixing until I removed / replaced the CPU. Evidently a bad
contact with the socket and the only time I've come across that :-)

Didn't think along that line one bit. Will open up the PC and do a thorough
cleaning and checking.

Thanks. Will give feedback how it goes.

maybe its hot, ck the disk utility.