I have an NT 10.5 having the exact same issue. Mine would shoot one round perfect but hot. When shooting consecutive balls, it would choke out and double feed causing breaks in the tip of the barrel - and it sounded wrong...
I replaced every o-ring in the marker, rebulit the reg but that didn't work. Yesterday, I removed the and cleaned the solenoid. Mine had some gunk build up. I don't have a pressure tester, but the market cycled after 5 turns - prior to cleaning the solennoid it required 7 turns out to operate. The few balls I had shot super straight and the marker sounded sweet. Dewll is set at 8 MS (I believe an NT 11 is different)
Not sure if this will help you - I will update you tomorrow after I get to the field and crono and cycle some paint.