miss fire at high rpm

hi all, i have a 1991 clipper 1800cc when i put my foot down it miss fires and looses power. mainley 3rd gear upwards at around 3000rpm any ideas what this could be???????

does it run ok after 3000rpm? if the misfire constand after 3000 or just at that rpm?

hi it is constant mate, seems to be worse when the engine has been running for a while!

Sounds like Knackered spark plug or maybe lead