GutWrench,
You seem to have a good 6P. Start with that. Switch both M2 lamps into the 6P and see what it does (looking for a bad lamp assembly). Then go back to the good lamp assemblies in the 6P and put on the M2 tailcap and see what it does. Since you have already eliminated the batteries as the problem, it has to be one or the other. You could also reverse the process and take the good 6P parts, one at a time, and replace the M2 parts.
Unless something weird is going on, it has to be batteries, lamps, or tailcap. My money is on a bad contact in the tailcap. I know that there HAVE been bad lamp assemblies, but to get two in the same box for the same flashlight would require a black cloud above your head. I HAVE had a shorted battery before but you say you have eliminated this as a cause. From what you say, it sounds like a low voltage problem from a bad contact point. The question is - where is it?
Hope this helps.