Most frustrating when you cannot pinpoint cause of the problem. Hopefully better brains than us will find solution soon..
It can be a benefit of the fault codes generated in the later cars such as the XK8 onwards – my 1987 Cab does have some codes (Lucas 9CU ECU) but literally about seven, read via the trip computer. I think the later 4.0litre had about 30 codes in the 15CU ECU..
Best of luck, Robin