+1 for checking for air leaks. The ECU will be affected by that. I the Haynes manual on the Rover 820 may help you .
This is the fault finding guide
Engine stalls
m Engine management system fault (Chapters 1 and 4).
m Vacuum leak at the inlet manifold or associated hoses (Chapters 1, 2 and 4).
m Fuel filter choked (Chapter 1).
m Fuel pump faulty or delivery pressure low (Chapter 4).
m Fuel tank vent blocked or fuel pipes restricted (Chapter 4).
Engine lacks power
m Incorrect ignition timing (Chapter 5).
m Engine management system fault (Chapters 1 and 4).
m Timing belt incorrectly fitted or incorrectly tensioned (Chapter 2,
Part A or B).
m Fuel filter choked (Chapter 1).
m Fuel pump faulty or delivery pressure low (Chapter 4).
m Uneven or low cylinder compressions (Chapter 2, Part A, B or C).
m Worn, faulty or incorrectly-gapped spark plugs (Chapter 1).
m Vacuum leak at the inlet manifold or associated hoses (Chapters 1, 2 and 4).
m Brakes binding (Chapters 1 and 9).
m Clutch slipping (Chapter 6).
m Automatic transmission fluid level incorrect (Chapter 1)
How old are the HT leads? The insulation could be breaking down as the car gets hot.
