Guiness.
I had a similar problem on my Disco 1 3.9, I had also changed to cats. Try disconnecting the ECU for half an hour, this will force it into it's default mode and remove the previouse data it is running on. Whilst mine is not perfect now (I think I need to look at the MAS) at least it does not stall at Rd juctions anymore when cold.
Good luck
Mike