A knackered rev counter WILL/CAN cause the car to cut out as it can give a permanent earth to coil so there is no pulse as such from ignition system anymore to coil just constant, so coils stops sparking, it's only upon the negative stopping that the coil collapses and issues a big current from the HT side, with no stopping of the LT side negative there will be no filed collapse and hence no current coming out, sierra rev counters are known for this problem, not very common but does happen to them.
tabetha