I cook them on my XL BGE @ 220 to 240 dome temp.
I start checking IT around 3.5 hours, give or take depending on how big the slabs are.
Once they get up to 180ish, I start the bend test every 10 to 15 minutes.
IF I have a crowd that wants "Fall off the bone", I cook them a bit longer.
I try to get mine competition, pull off "doneness" every time.
I cook mine bone side down, platesetter in, legs up and on the grate. I also use a steel drip pan on top of the plate setter. The only foil that touches my ribs are when I pull them off to let them rest.
I get a 3 packer from Sams for around $28 to $35...depending on the size/weight.
Did you pull the membrane?