I would keep the vent cap at 1/4 open and maybe light 2 less coals next time. 335 isn't bad for chicken.....I think it just got too hot initially and couldn't recover fully.
Also, once you are able to get a stable temp with your vent cap set, get some magnets or use a leather glove to partially block the intake if you get a temp spike and need it to come down. Doesn't happen often but it'll drop quick when you choke it down some.
How'd the chicken turn out?
It was easily the most moist chicken I have ever had! Wasn't crazy about the PBC All Purpose Rub, so maybe Yardbird or Kosmo next time. Ribs planned for the weekend (beef and pork) so I'm going to really dial down the vent at first. 300+ is just too hot for those.