Tim
First thoughts: Since the lift channel works ok in some setup there is nothing wrong with the ESC or Motor
Ditto the thrust motors
So I suspect strongly that you have some mixing built in to the Tx setup that is having this effect - the #6 is somehow mixed with the thrust channels - I would guess these are 1 and 2 and these are aileron mixed? to give you forward/reverse on the throttle stick and left right on the ailerons ?
If you are happy to have the lift completely independent on #6, then it must be completely unmixed with any other and just twiddeld off the #6 control whatever that is
If you tell us which 2.6giggle set it is there will be a stampede of experts helping with setup.
If it is the Turborix I understand that #5 and 6 come from the factory un-connected (ie its a 4-ch basic set as delivered) and have to be woken up with a link to a computer, but thats not likely since you have had #6 working.
andrew