You say "power from the breaker is "Fluttering" which should not be possible but.. Read on.
Are you sure it is power from the breaker to the Magnum and not the Magnum that is the issue? (Hook a volt meter up where it can tell.
Assuming it is as you said Power TO the Magnum. not from it. The suspect list:
Bad breaker (I know you said it is new. but that don't mean it is not bad).
Breaker not fully seated
Loose screw.. Breaker or White bus bar (odds are the breaker screw is tight since you replaced it)
Broken wire breaker to Magnum
Loose connection: Magnum end of wire
After that the inverter itself becomes suspect.
