OK, I will go by this next time. So just to confirm what I am doing for my baby backs next time:That thermometer was your problem. For ribs, keep the thermometer in the drawer. Ribs is done when they're done.
If you're doing baby backs, you can foil when you feel you've got good bark and smoke (3-4 hours at 225F) Spares are forgiving and will be more moist, so you can easily go straight thru without foiling at temps from 225 to 300F. Just remember that a pellet grill makes less and less smoke as the temperature goes up. St. Lous at 225F could take over 7hrs. to get tender.
Tender is what you're looking for. Your first indication is the rib bones start sticking out (meat pulling back). I grab them and try to twist them a bit, pick up the rack (gently) and see if it's really flexible. The bones should twist a bit out of the meat. After a few times you can get a feel for it without pulling a bone out, but just moving it side to side and flexing the rack. For your first few, go ahead and pull a bone or break the rack.
Once they are tender, you can think about putting sauce on them and let it "tighten up" on the smoker for 15-30 minutes.
1. Cook naked for 3 hours
2. Put on foil with juice.
3. Cook in the foil for 1.5 hours. Check ribs for tenderness as instructed.
4. If not ready, check every 30 minutes (Or how often?)
5. When ready, take off the foil, put on BBQ sauce and throw it back on 15-30 more min
Does that sound like a good plan?