All your possible causes seem valid. How about loose rudder surface? Flat turn at the moment of lift off, if I understand this? If all else you proposed and no glitches in the radio, the lack of effective rudder seems like a good one. Got a spare gyro around? Put it on the rudder and see if the problem goes away..it can react faster than you can.
Just a thought.
twinman..
PS...you got that four engine monster ready yet???