1200GXman
Don't just tighten the flywheel,.. remove it. While it's off, carefully examine the inside of the flywheel where it touches the crank, all six bolts, & the crank itself. Check the fit of the flywheel on the crank. There should be no looseness.

If you are completely satisfied that everything is OK, then refit it & tighten it.
If it HAS been moving, the bolts may be damaged & retensioning them without first checking them could be bad news.
If they ARE damaged, & you are lucky, then one or more will break while you tension it. If you are NOT lucky, the damaged bolts will not break untill after you have it back on the road.

Before refitting the bolts, make sure that the holes, & bolts, are sterile clean, then use a little of the correct grade of Locktite on them & do them up FT [the "T" is for tight], check the manual for correct specs.
My workshop manuals say 54 ft/lbs [1200] & 61ft/lbs [120Y] I seem to remember that with new bolts, i went a bit tighter than that. The number 70 seems to ring a bell, but don't quote me.

I can not nominate the rpm's where these harmonic frequencies are at their worst, but race engines suffer this problem most, so it would be in the higher range without doubt.
I would highly recomend that you educate your right foot untill after you have pulled it down for inspection & repair.

Balancing of the whole rotating, & reciprocating mass, including the clutch pressure plate, is HIGHLY RECOMENDED for ANY engine that will regularly visit the higher rev range.
Good luck
Chris
P.S.
A harmonic ballancer will help this problem, but there are none available for A series engines. My mate with the injected A15 in his 1200 coupe racer has shed a couple of flywheels, so he adapted a falcon six balancer. It isn't pretty, but he hasn't shed a flywheel since, & this thing gets hammered hard enough to win the season championchip,....again.