rover 414 1997 wont start?

my car was running fine slight judder every now and then but one day rev counter stopped working parked it at home next morning it started i left it to idle and it stopped and would not start i called the recovery people they managed to start it and it died again he tried again and it did the same he said i must replace the crank sensor and ignition coil as the car is loosing its spark i have done this and it started and run for about 20 min it stopped again and it wont start can anybody please help me