I agree with what others have said and I would start with a vacuum gauge connected near the manifold to check for leaks at different RPM's
There is one other thing I would also try and that is adding one quart of Marvel Mystery Oil into the crankcase.
I had an intermittent miss on my 350 that drove me crazy!!!
I changed plugs, wires, vacuum lines, distributor and everything else I could think of that would cause a miss.
An 88 year old friend of mind who had worked on engines his entire life told me to try and Marvel Mystery Oil because he said I think you have a lifter that is sticking just some of the time.
Long story short I tried it and never had the problem again.
Hope that helps. Jimbo