Not doubting the quality of Bobby's strings (actually never saw one as far as I know), but it doesn't matter how well made it is, dacron (B-50 and B-500) stretches--that's a simple fact.
With my jig, I pre-stretch the strings under 250-300 lbs of torque (I've broken 10-strand dacron strings on my jig). No matter how much you pre-stretch, it will retract--even Dynaflight, 8125, and 450+ have some elasticity, although it's nowhere near as much as dacron.
DCM, other than using it for old bows, dacron is good for padding the loops on the newer materials--seems to help quieten limb slap a little.
I also think you nailed it about tuning--I know I've been guilty of swapping strings and not taking the time to tune it properly, and then get aggravated when it was noisy. Takes a little effort on our part.
Draw weight, leaving it on the bow or unstringing, temperature (the hotter it gets, the more it will stretch), number of strands in the string, etc. all have an effect Pre-stretching does help a lot, but it won't keep dacron from stretching.
Almost forgot--Bill, I have tried an endless loop dacron on my recurve, trying to get a fishing arrow to fly better (I make endless strings as well as flemish). I still prefer flemish Dynaflight. :D
Chad