Dany's Magnum swap is back!! with Ignition woes now..

If your engine has hydraulic lifters;
and it breaks up at ABOUT the same rpm, unrelated to throttle-opening, then you are likely looking at valvesprings or possibly an ignition coil.
If at exactly the same rpm, then likely the amplifier.

But if she breaks up at various rpms that can be directly aggravated by the throttle opening, that is most likely spark plugs, coil, or fueling. But if at mid rpms, then NOT likely to be ignition related.

So, if you can drive thru the breaking up, by reducing your throttle opening,I'll guess your plugs are cooked. I would install new colder plugs and probably enrich the related fuel-circuit.
But if you cannot drive thru it, and the engine rpm goes flat, then I would check the valve springs, or the mufflers.
But if the rpm does not go flat, forget the valve springs, and check your gas for water.

But if you have solid lifters, check your hot-lash.
That's the best I got.