engine cut out

Hi Guys - Can somebody help me please ?
I have a 1994 XJ Limited which has started to cut out while driving. It accurs at various speeds and is intemittent. Usually you can start the car immediately, other times it may be 5 minutes. The engine will crank over but just won't fire up. Had a mechanic find that there was no power to the coil but then it corrected itself before he could test it further and it now refuses to go faulty if a mechanic is within cooee (Aussie slang for near) of the car.
Hope you can help.
Cheers Chappy

put in a new coil

The same thing happened to my '92 XJ two years ago and it turned out to be a bad CPS (crank position sensor). It's a common problem on high mileage 4.0's and the cure is to replace it.