Hi I have just had my 06 gear box re built so still under warranty, but I have noticed when I slow down and I am doing about 20km an hr there is a noticeable shunt but this only happens with the air con on, I have driven with it off and in sport mode and it behaves normally. I am hoping it is an idel issue, can anyone shed any light on the problem. The rebuild has been done by RR specialists. It will also be going back to the shop on Monday.

Your 06 what ?
Aircon changes idle speed and you may need to reset adaptions as a guess.
There was a software update for some ZF transmissions I believe to stop the Californian rolling stop or something , can’t remember exactly but basically throttle off and back on without coming to a complete stop.
I may not be able to see your vehicle profile on my phone but an 06 could have a a few variations of transmission.

Sorry L322 06 and I have the gm gear box,

I know there is a problem with some vehicles where you get a sensation of being hit from behind by another slow moving vehicle when coming to a stop but but as yours only does it with the aircon on then sounds like it’s idle related, does yours idle higher when your a/c is on ?
I’m not sure if it’s a good idea to reset adaptions but somebody like RRPhil would give you a much better insight into what might be going on.
Do you know if the valve body was reconditioned and tested when the transmission was rebuilt?
With that transmission I would definitely want the Sonnax upgrade to the valve body as wear in that causes the failure of the transmission in the first place.
Have a read here as a harsh downshift can be caused by the body.
You have the GM 5L40-E transmission.

https://www.sonnax.com/parts/2191-oversize...-valve-kit