Your symptoms sure sound like your MB is overheating -- even though your CPU appears to be operating within normal temperature limits. It might not be the CPU that's overheating -- it could be the GPU instead. The fact that you hear a fan right before the lock-up sounds like it's temperature-related.
Try increasing fan speeds with SMC Fan Control, run with increased fan speeds for a while and if that makes a difference. If it does, then we know your problem is temperature-related.
Link:
Controlling your Mac’s fan speed with SMC Fan Control (with step by step guide) « David’s logbook