i've only done 1 butt, but it went backwards shortly after it stalled at 169 degrees.
it didn't freak me out because i had read lots of threads that warned of the stall, and a few that mentioned the loss in internal temperature.
old threads are a great resource here at SMF. if you root around you can find the thread where someone (sorry i can't recall the member's name) explains the physics behind the stall. it's very reassuring to know what problems might crop up and have strategies ready to addresss them.