Well something is out of balance, that's for sure. Not sure what you mean by the "usual" checks, but whatever you didn't check is likely the problem (haha?). You may be right, since too much play will also present itself in any rotating machinery as vibration.

Sometimes you can get a clue by studying the heads while running up the heli while holding it in your hand. Let it spin slowly and see if you can tell what is wobbling off-center. You know it could be something as simple as loosened upper head set screw that allows the head to slip up slightly from the upper bearing, allowing the bearing to float and wobble.

You would think it would be related to vibration induced loosened screws and fasteners or something like that or like you said worn bearings.

If it were me and I was sure the shafts are true, I'd do a general tear down and inspect everything. That being the case I'd have new bearings in hand before doing the tear down and replace them regardless. The upper inner shaft bearing could be the culprit I suppose.

Since you've obviously done this before it should be no biggie for you to do the tear down.

Let us know what the cause is when you find it.

Mack