UK Car Repair: 1996 Jaguar XJ6 misfire, transmission locks, 1996 jaguar xj6

Question
My XJ6 is not runnig cleanly and will sometimes stall when idling.

After it stalls, the check engine light will come on and sometimes the transmission locks in the third gear "limp home" mode. Restarting the engine will clear the transmission lock but not the check engine light. My mechanic says it needs a new ECM (we have tried a new crankshaft postion sensor) at a cost of almost $4,000!! for the part only.

Any ideas and if it is the ECM where I can buy one?

Answer
Do you mean FOUR THOUSAND? that is impossible, the sensor. DBC 10603 is $250 US. Furtheremore a Crank position sensor will not cause the vehicle to run rough.  They almost always fail at a stop preventing starting!  Has the computer been scanned for fault codes? It really sounds like you need to find anothere mechanic.