I was just gonna ask the same as Swindon Ed regarding the membrane, Your 20 mins resting needs to tripled to at least an hour ! I always Foil my ribs, and their tricky to monitor the cooking temp of the meat as the bones read differently to the meat between them so your thermometer placement is a tad hit & miss so with ribs I have to peek under the lid every hour or so to check on them, Sounds like yours might have been over cooked and slightly drying out, I usually rely on a good mopping everytime I peek and 'ding' my mopping sauce or beef stock so it's hot when I apply it to keep them moist.
I nearly always always let mine marinate in their rub overnight, I'm sure the salt content tenderises the meat.
G