Cranksaft position sensor failure?

[Niall]

So it turns out i haven't fixed my car. Fitted a new coil pack over the weekend and it was running a lot better. Took it for a brief drive and seemed fine. Got the car out today, went to get fuel then just passing my house to head to ace and bam, dropped off firing on cylinders 1 and 4. Im not convinced its a wiring issue as when it decided to break down at the side of the M1, it was sitting there dead still on all 4 one minute then drop to 2 then 4 as it felt so not like any particular load moving the engine pulling a cable etc.
So, I've eliminated coil pack as I've fitted a new, completely different one. It COULD be the ECU coil driver but this seems doubtful as 99% of the time, they just fail completely. Also, this only seems to happen when the cars hot and the ecu is in the cabin so not effected by heat. Ive been told it could be the CPS but not sure so wondering if it sounds right to anyone? The car has always taken ages to fire but i would imagine if it had failed, it just wouldn't fire at all seeing as all it does is pick up on the missing tooth? Or am i just over simplifying things?

Cheers all!