This is a bit of a puzzler, dirt kicker. Any chance your propeller was chipped or damaged at all since the previous engine run. Based on your description, an out-of-balance prop is the only obvious possibility that comes to mind.

If you were running the plane on the ground, it's not impossible that the carburetor could have sucked up a bug or some dust or something. I could see where this could lead to the kind of misfiring you described.

Lastly, it might be worth trying another fresh glow plug. A brand new glow engine will tend to shed some metallic particles in its exhaust while breaking in. These metal particles can foul a glow plug fairly quickly and lead to poor idling. A lot of veteran RC pilots will advise folks to, "always break in a new engine with an old glow plug" for just this reason.

Based on these possibilities:

1) Check your propeller and verify that it is still balanced and undamaged. Spinners can also be out of balance, so if you've added a plastic spinner since you last ran the engine, this too might require some checking.

2) Verify there is nothing plugging up the carburetor or needle valve assembly, you can visually inspect and also flush the engine out by pumping a little fuel backwards through the carb and the needle valve

3) If neither of these issues seem to be causing your problem, go ahead and try another glow plug. Start with another used plug if you have one that is trustworthy, or use another new one if you don't have an old one that you know should work.

Good luck!