sounds like a miss to me too.

some engines do this - technically it should be okay free-revving and holding steady, but generally i have found that (A12s especially) don't like being held at 5000rpm with no load.

let the clutch out though, and the miss disappears.

it could be dizzy float, it could be backlash in the dizzy gears, it could be your points - geezus there's a million things it could be. it could even be as simple as your coil is breaking down or a lead shorting on the rocker cover.

engines aren't designed to be free-revved so chances are they won't like it. rev it to 5000 in first and hold it there and see if it smooths out. if not, then check for problems.