Misfiire on 1996 Jaguar XJ6

I have a 1996 Jaguar XJ6, six cylinder, not supercharged, with 110,000 miles.

The car would stall, the check engine light would come on and the transmision lock in thrid gear. I could unlock the transmission by restarting the engine but the check engine light would remain on, car would run rough and stall when idling. When it stalled again sometimes it would lock the transmission but not every time.

We checked the plugs, plug leads and cleaned the injectors.

My mechanic diagnosed this as needing a new crank position sensor which was fitted and it ran worse.

The next diagnosis showed a problem with the ECU. We had the ECU rebuilt and reinstalled. It now runs better but still misfires around 50 mph and almost stalls at idle.

Any suggestions?