172 dropping a cylinder

[hoperacing]

hello guy's.
my mk1 172 keeps dropping a cyinder when you start to make the engine pull hard. after 5 - 6 thousand rpm in fourth it will drop the cyliner furthest away from the cam belt.it will then run on three for however long it decides to. sometimes it will clear in minutes. others i will park it in frustration for a week, go back and its fine. it has had a new set of plug's to try and solve it and i have swapped plug leads but problem remained with the same cyliner. no ecu light on the dash either!!
anyone got any idea's?

 

[Munson]

Have you had the distributor tested?

 

[hoperacing]

no not yet. are they expensive to replace? i thought the most likely option woule be injecter. when you pull the lead off the dustibutor with the engine running the spark between the two is strong but as you move it so far away that the spark cannot travel it makes no difference what so ever to the way the engine is running. when you remove any of the other it drops onto 2 and just about stalls hence why i know which cyliner is failing. just can't identify why!!!
really bugging me as you could imagine.