I'd be sending it back to the seller on the basis that it doesn't perform as it should... or get it checked again at an overhaul facility at the sellers expense. If they can get it running problem free on a test rig then it is eliminated from the equation and your problem will be elsewhere. I get the impression that the seller is laying low and hoping you will just go away.
Being what it is I would want to be 100% certain that FCU is good to go....via an overhaul facility.