The ol' Century 22 can't seem to get up past 390 when on high and I'm wondering if the stack cap has something to do with it.
I seriously doubt it, but what have you tried??? If you tried screwing it down flush and opening it up all the way and it's the same then I say NO it's not the cap. Did you remove the cap and see if there is a blockage?
I mean the 2 extremes would be take the cap off and try it... screw the cap down level and try it... that's the only choices you have.
Not to ramble on here but I don't care what that guy's Youtube said, I will have a hard time believing the 'cap' is a big temp 'tool' when it comes to a Traeger. Even tho I made a MOD to slow my heat/smoke exit, since making that and having my lid level with the tube top, I think mine cooks the 'top' of my meat more now.