help,
have a 99 flht ,have replaced both wheel bearings and races, have replaced neck bearings and races. have installed upgrade swing arm bearing kit , have installed rear stablizer kit . Bike does fine up to 75, from there to 90 it has , the best way i can discribe it is like riding in a bad wind storm,the handle bar shifts from side to side ,but not a wobble and not all the time it comes and goes but very very quickly. Not something you could take one hand off the bars!! Both myself and the local HD shop are puzzled anyone every have a similar problem and did you fix it!!
Ragu

 

There are a few more things that come to mind worth checking that you didn't say you tried yet. Did you take a string and check that both wheels are tracking absolutely straight together? You really have to use something like a straightedge line to verify the tires are lined up. I don't trust any other method myself as too much is riding on your alignment.

Did you experiment with the windshield to see if perhaps it is magnifying any sudden wind gusts at speed? Try adjusting angles or take the darn thing off and try to repeat conditions over the same strip of road that causes the problem. Shields are a mixed blessing. They keep you comfy but they can cause handling maladies.

Did you experiment with tightening the steering head neck bearings a little bit more than the book? Or are they too tight and not letting the bike settle down?

Have you ever considered that your bars are too wide and you are inputting too much steering input which starts an oscillation? I personally use pullback Z-bars that look like a cross between drag bars and clip-ons, and they are only 24" from one tip to the other (very very narrow). These feel nice and heavy at speed and stay secure at 100.

Have you checked that both fork legs are holding the same amount of oil and have the same resistance? Is one leg shot? Anything worn out with the rear shock setup?

Have you spun the wheels and looked at runout to see if the wheels wobble?

Last of all was it a windy day? All bets are off if a cross wind is gusting. I tried riding in a 30 mph crosswind in the mountains and it was literally moving me across two lanes at will...not fun. Nothing wrong with the bike.

Maybe somebody else will have even more stuff to chew on. Eventually I am betting you will find the bloody problem. Don't give up until you can do 100 any damn time you feel like it---except in a wind gust haha.

 

What about wheel balance? Alighment needs checked for sure. Was it a used bike? A wreck could mean a bad frame...