2 and a half vents were open
Something is wrong with your thermometer. If I was running 8 lbs of decent quality lump with hickory chunks buried within he lump. and 2.5 out my 4 vents open, I'd be running over 400 degrees.
With lump, in a UDS, I normally can cruise @ 275 forever (at least 12 hours). I have four 1" intakes in my UDS covered with fridge magnets. It would only take 1 intake maybe 3/4 open to maintain 275.
Definitely something with the thermo, or placement of thermo.
Bad lump (Cowboy) may be another contributing factor.