Cheers for the replies, I don,t think it is anything to do with the distributor, but does the rpm sensor just send the signal to the ecu, where is the rev counter picking up from.
Also the blocked cat maybe a possibility, it happens dead on 5000rpm every time, I find it strange that a sensor may have failed but still manages to get the ecu to cut the engine out at lower revs, the rev counter in the car is reading correct.
Jim