These Evolution engines like hotter plugs than the OS 8. That is most likely the problem. A hotter plug turned mine into new engines. The Saito SS plug is a very good one for the Evolutions. You will gain much more power and it will run way smoother. I ran an Evo .455 yesterday after a year of not using it. Put a SS plug in it and a Tower's muffler on it for the first time. Wow. The .46NT is a great engine too.

If you have any of the older type of Tower's mufflers, use those as they are even better. The newer ones from Tower seem hotter than the OS 8, but not as hot as the older plugs. The new ones work well if you don't have the Saito SS plugs.