340 vibrates

Think I just figured out a little of mine.

Went to the garage and took the fan/alt belt off.

Fired the car up and let it warm up a bit. (was already warm from my drive at lunch time).'

So just sitting there in park, I revved it up to 2000, then 2500, then 3000rpm. (guessing since I don't have a tach). Sure enough, the engine is stumbling and missing at these steady rpms.
I thought the car was running good, starts easy, idles good, runs good under hard acceleration. But apparently when I'm at a steady cruise rpm, the engine is running like crap.
So now I have to figure out if this is a carburation problem, or bad wires / coil, plugs ?