Crank bolt coming loose

Hi everyone

I've recently experienced the driveside crank bolt undoing itself while riding and I'm interested in people's experience, thoughts and recommendations about the issue. I'm riding a HT 29er with Shimano M430 Octalink crankset and Shimano ES25 BB. The bike is just over 12 months old. The issue first came to light about 6 weeks ago when I noticed some play in the cranks during a ride as well as being unable to shift into the big chain ring. Got home, discovered the loose crank bolt and re-tightened. Subsequent rides were fine and post-ride inspections didn't reveal any play or other issues. However, last weekend during a 12 hour adventure race, the bolt came loose on 2 occasions. I re-tightened the bolt during the race, but by the end of the event, the bolt was really loose with a lot of play in the cranks. I've since removed the cranks at home and completely de-greased, cleaned, greased and re-installed the cranks. The value range on my torque wrench isn't high enough for this component, so I've only ever hand tightened crank bolts "as hard as I can" There was no visual sign of damage to the splines in the cranks or on the spindle and the BB itself seems fine (ie: no tightness, no play, no "grinding" noises). The threads in the cranks and on the spindle also looked ok. The bike gets ridden 2-3 times a week and I do enduro mtb events and adventure races on it. I would describe it as regularly and well-maintained. My plan is to hit the trails this weekend to see if the problem repeats itself, but I'd really like to hear people's thoughts or experience. For example, is it time to think about a new/higher spec crankset?

Thanks in advance.
Paul