P0281 and hesitant cold start

[Collin02]

Hey guys, looking for some help. I have an 05 6.0 with 176k on it and I'm having this problem mainly when the engine is cold, regardless of temp outside. When I go to start it it cranks fine but hesitates on first start up then smooths out. Also if the engine isn't completely warm and your driving you can tell it's missing in the higher rpms. I checked for codes and I'm getting p0281 which is cylinder 7 imbalance or something like that, tried using archoil but didn't seem to make any difference. I think it is probably an injector but I just bought the truck 2 weeks ago and I just want to make sure that's the problem before fixing it. Any help is appreciated! Thanks