It's not just some thing silly like cross firing on the leads is it. Do you know which one it is, cyl wise ?. you could maybe track it, if you don't know. Just do it like a jet check on the plugs. Hold it on the mis at revs on the street and just shut it down, push the clutch in and coast to the side of the road, pull the plugs and have a look at them, maybe able to tell which it is assuming it is just one. Don't be tricked by "I ve already replaced that piece either" It's real easy to instal a new crook part. I looked at a V12 HE XJS once, that everyone had played with, it had new ECUs etc.ect. Fault was the first guy fitted plugs for a Pre HE engine, which are short reach, HE's have different heads and different plugs, so it would idle, run at lite throttle but fall over if you got on it. Thing is it takes a few hours to change the plugs on a HE engine due to the head design, everyone knew they were new, so no one pulled them out to checked them. Not saying it's your plugs in this case, but you know what I'm getting at. If you can find that it is just one cylinder, I'd then go from there, could even be a weak valve spring, I've seen that on ski boat engines which can often get buzzed too high, once you float the valves the springs are toast, quite often. Only takes one weak one to drop a cylinder at high revs, just float a valve at lower revs.
