I did 2 of these last weekend. Each one was 2.5 pounds. Mine did the same thing. Thought it would never get tender. After 165, foiled and went to 350, total of 7 house smoker time. Then I put it in the oven at 400 for another 1 1/2 hours before we could eat it.
Performer, UDS, 2011 WSM, Frank-in-Weber, 120 Gallon Offset, IQ-110, ET-732