Brendan

I have a Tokyo Marui P90 with stock internals except for the barrel. That is an element 6.03 mm tight bore. When i am using a low cap magazine, or hand loading the hop up the device shoots as expected and consistently, But when I use my JG box mag the p90 will be shooting fine one moment, then skip a round or two. Sometimes fire 2-3 rounds. Or have major fps fluctuation, where it will drop from 350- less than 100 fps. My thoughts were that i need to replace the hopup, but what still puzzles me is the fact that it is working with everything but the drum mag. What parts might i need to replace, or what can i do to fix this?