I'm with Smoke, I don't use my PBC for low and slow cooks anymore, mostly chicken and sometimes pork ribs, but when the PBC was my primary cooker, I used to do low and slow by controlling my temps with bits of aluminum foil wrapped around the rebar holes. It took a bit of tweaking and an accurate digital thermometer within the cooking chamber, but I could dial in pit temps of around 250 for hours at a time.
My guess is the higher temps and quicker cooking time could be part of the issue, and as we all learn, beef isn't done until it's done. Maybe wrapping with some liquid and finishing the cook on the cooking grate might be an alternative?