Not sure if you fixed this issue but I had exactly the same problem - turns out my magnecore kv85 leads were the culprit. I had assumed being advertised as high end items they wouldn't break down after just 9k miles but the coil end had disintegrated
and 2 out of 6 leads would change impedance dramatically - from around 5k ohms to over 300kohms - when you waggle them. The guy on the phone at magnecore was condescending when I raised it with him and refused to believe their leads could possibly fail. It's all the customer's fault !
Changed them for 8mm leads costing 1/6 the price and the car runs sweet as a nut again.
https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F181999641503
Check / change your leads too and see what happens.
