Gonna have to go with Shawn on this.
When electrical current gets passed through the wires and contacts they heat up. When things heat up they expand. What could be happening is that there is a less then optimal contact or solder joint somewhere (probably a solder joint) and it is expanding when the string gets hot. Expanding enough to cause a break in contact which would cause the lights to go out. And then when the circuit cooled it make contact again so the lights would pop back on. Until it got to hot again and they'd go out.
This kind of problem can cause you a head ache because you can test the fixture when it's down and not running and everything will seem fine. It's only when things heat up that the problem occurs. So it's going to be hard to test without running the string.
What he'll want to do is take the fixture down. Wire the moonlights back up to the driver and then let it run. Wait for the leds to kick off. As soon as they kick off while things are still hot then run a series of continuity checks down every led on that string. He should be able to find where the problem is that way.