Hi I am not sure if this helps but if you are still encountering the same issue can you try to see if the DVD drive still works correctly after the shutdown? I.e. read a disc.
I have just replaced my old late 2008 Uni-body MacBook Pro because it would randomly put itself to sleep. The fans would be running on full power and the CPU temp would be through the roof ( I was monitoring it with Istat Menu meters) also it was plugged and charged so that was not the issue.
I could tell when it was going to go to sleep as the connected external discs would sleep first and then the machine would freeze and then go into a forced sleep.
Tried numerous SMC fan resets and also a PRAM reset, which should only be done in the most extreme issues as it is apparently quite dangerous or risky.
Turned out there was a fault in the Logic board and that it was not maintaining temperature control and thus the MacBook Pro was getting too hot and shutting down to protect itself, but not before taking out the DVD drive!!
Which echos the comments made above that it was a Logic board issue.