Need to replace Gearbox for my N42B20

[Kelvinopt]

Need advise,
I just replace a secondhand gearbox,it keep coming out whining & roaring sound somewhere around the gearbox.Especially low rev press gasoline less response and can rev over 2500rpm on 1st gear.The gear seem keep changing itself to 5th gear on short flat road.Climbing hill the gear itself set to 4th gear.On & off I shift to steptronic to gear the gear position.The car seem can't kick in when trying to press on gas pedal.
Any advice?
By the way,I already change to Mobil 1 fully synthetic ATF Multi-Vehicle Formula.
Very upset.Anyone can advice whether need to replace again a secondhand gearbox.
Where I can get a used japan chop shop gearbox in Penang?
Why sometime I slightly press on gas pedal,the gear lag a second with a sound "tok",then only
Can get kick in to rev.And after a few high rev,the gear downshift from 2rd to 1st will jerk/strong vibration.

 

[tweiboon]

Jerking might due to the valve body solenoid valve. They replace whole gearbox with the valve body ? Have you take back your gearbox ? It also might due to internal clutch of the gearbox. See which 1 problem you are facing.