[DuckNut]

Look down in the list for the post from TomFL titled OK, I'm stumped and you may get some ideas.

 

[firecat1981]

Probably not a spark plug issue if it was it would usually run ruff through out a wide range of RMPs. Might be a carb issue, when was the last time they were cleaned?

 

[aflatsnut]

You might also be over reving the engine and rev limiter is kicking in. What RPM are you at?

 

[firecat1981]

Wait, is this motor new to you? or have you had it, been running it for a long time.

 

[anytide]

check timing advance , vacuum or computer driven ??  -could cause to break up ..   as earlier rev-limiter ??

-mis fire, run funny  -  fuel filter @ wot = run funny ?
                             mis fire @ wot =  timing ?
                                                   -anytide