The bars keep loosening/rotating on my son's road bike. This has happened a few times but Friday with near catastrophic results on a downhill. These are used craigslist bars I installed. When I installed them I did not see any obvious signs of damage. Plus, damaged bars don't fit the symptoms as far as I can tell.

details:
FSA wing carbon bars, 31.8mm clamp area
Easton EA70 2-bolt alloy stem (with a substantial faceplate)
assembled with carbon assembly paste on bar clamp area
I greased the threads of the faceplate screws (bearing grease)
5Nm torque

Symptoms:
The screws seem to be backing out. Yesterday I checked his bike by dialing back the torque wrench and there wasn't even 2Nm of torque on these. I tightened to 5Nm and we went on a ride (with the torque wrench!). In less than an hour I checked again and the bottom screw was slightly loose. By the end of the 2-3 hour ride, they were holding fine.

I was wondering about the grease on the threads. I asked a couple of my LBS mechanics and they both said greasing them was the right thing to do. And wouldn't guess any further without taking the bike into the shop.

I was considering removing the grease and using Loctite.

Something I'm missing here? Any ideas?

edit: one last thing. I've got a 4-bolt ITM stem (with an even larger faceplate) I was planning on swapping in place of the Easton. One of the mechanics blessed this idea. It doesn't address why these screws are backing out, but it seems like a prudent thing to do.