APS UAR issue

[stormcloud_59]

When I fire my UAR it will shoot fairly well and then all of sudden it lets about 3 or 4 BB's just sort of roll out the end of the barrel. If I clear my mag and reinsert it will function fine for a bit and then repeat. Could this be a bucking issue?
Secondly when I auto fire it will shoot and then every so often it will stutter almost like it missed a beat. Then continue to fire till I run into the above problem.
I have replaced the carrier inside it and now it will fire midcaps so that problem has been fixed.
I really like the look and feel of this gun but need to make it work properly