System is shutting off without warning.

My Macbook Pro is shutting down as if someone pulled the plug out. The laptop is plugged into AC and the battery is fully charged. The most recent event I was working on a Photoshop file... system was only running at 30-40% of max and the temp was around 145f when it switched off. The system reboots fine and I cannot find anything in the log to indicate a system or software related event occurred.

This has happened 4-5 times in the last 3 months... 3 times while I was away from the machine (overnight etc.).

Do I have a bad termister sensor that is shutting the system down or is this a power management issue?

Note: The AC adapter was replaced a year ago. The battery was replaced 6 months ago 100% health. I pulled the system fans to ensure that the cooling system was not clogged with dust checked 4 weeks ago (it was clear).

Not sure how to approach the problem as I can't duplicate the event to determine the cause.