Simply put i have a rev counter which goes up and down 50-100rpm while driving at a constant speed around 60mph.

I've read a few things now about torque converters and gearbox updates.

Weirdly though the car was all good last week and this started on Monday morning. At the weekend I found a vacuum leak whilst at a mates garage and we replaced the damaged hose and all was good on the drive home. On Sunday I changed the clock spring and had the battery off for a number of hours that day. 

Do you think the hose replacement and battery being disconnected are linked? Or is it likely that the Gearbox does have an issue.

To add: S-type 3.0 V6 Sport auto 2003

 

...and I does seem to clunk into first gear when slowing down 

Little update... from what I keep reading up about it does sound like the torque converter so I have sourced another and have ordered ZF gearbox filter and oil too. will post back

I did a silly thing on Saturday, I dismantled and cleaned the throttle body! After 98k the sides were coated with a thick oily residue. Amazingly, the engine characteristics have changed. The transmission is smoother, it revs freely and it starts with a roar. I think this is because the idle/low rpm was rich before the cleaning! So there are several reasons why the ZF can give odd running.