One more thought, besides the probe Dave mentioned. 5 lb butts are a little unpredictable, so to be in a stall at 160, after 7 hours, is not totally unheard of. How, again, did you measure the box temperature? And was the probe near the meat, or back near the box probe?
If you reliably measured the box temp, it could be possible your controller rheostat is bad. If you test the box temp, with nothing in it, see if it gets to 250. If it doesn't, contact Steve. It could be a bad controller and/or element (yes, it's possible to have a bad element out of the box). Call Steve. If your box temp readings are accurate, something's not right. My #1 (er...my sister's #1 now) has never failed to reach set temp, no matter how much I stuffed in it!