hi,
Have you reset the base idle speed, I had a similar problem with a disco a few years ago and the very nice and helpful Bruce at Camco88 (Warrington)advised that I should check and reset the base idle speed (BIS), he went on to explain that if the BIS is incorrect then the engine management gets its knickers in a twist trying to stabilise the engine, it cant so it ends up all erratic..
as far as I can remember bis is 650 ~ 700 rpm and controlled is @800#, I snceet mine by referring to the workshp[ manual and it made a big difference,
I may have a spare throttle position potentiometer if needed.
Regards
Phil