Have you done anything with the Bellville washers?
I cleaned and re lubed the bearings, and before I put the washers back, I polished the mating surfaces with 1000 grit on a granite surface. Then coated the surfaces with anti seize. I also think they are a bit tighter than before. I used to get vibes at various rpms but things seem much better now.
Check bolts on retaining ring at the holder end.
Maybe even disassemble top to retorque upper bearings retainer nut. Do washers at the same time.
Can’t think of anything else…