Found this on-line. Might be useful to someone..
Steering Shakes, What The Manual Doesn’t Tell You:If your bike steering “shakes” when decelerating from 40-45 mph and less, the steering bearings may be the problem. Uncontrolled lane changes, wobbles, and general control issues not caused by the rider (on either acceleration or deceleration) can be caused by many factors. Let us go thru the causes and rule out all but the defect. Look For:• 45 mph or above: Look behind the steering head.• Under 45 mph or so, look to the front of the steering head.All cases check: tire wear and balancing, loose spokes or bent mags. The tire may be defective, a belt could have separated or numerous other things. Stuff you can not see. Try swapping whole wheel assemblies with a similar or stock true tested wheel for that year bike. Check out your motor mounts. This can be a source. The newer model FL's are known for bad motor mounts. There was a run of defective and incorrectly manufactured units installed and distributed. Make sure you buy the new updated motor mounts. If the speed is under 45 mph and the wobble happens and your wheels are good: Step 1:Look at the stem (steering/head) bearings. • Jack up your motorcycle so the front wheel is off the floor grab hold of the forks and move the bottom of the forks in and out; listen for a clicking noise. Next turn the bars side to side, if the wheel self centers or is “notchey”, replace the stem bearing. No brain surgery here!• If you do, remove, buy good quality new bearings (they are still cheap), repack and replace.If you are just replacing the bearings and you are satisfied this is your problem move to step 2. I would advise reading further and check the next while you have the bearings part. Do not be lazy, replace the races!• If you do not hear a clicking noise; remove forks & triple trees check to see if the bearings themselves are loose (now we are not talking tightening a stem nut looseness). This is the interesting part. We're talking about something that is not addressed in any service manuals, factory or aftermarket. Whether because the frame is made of softer material, a cheap custom frame, bad build or what, the steering bearing races could "walk" in the frame. That is, they shift in their recesses and become out of square with the steering stem, and out of parallel with each other.• Steering bearings tend to "minutely vibrate" -- shift back and forth -- in their frames, especially on the heavier bikes. The resulting non-parallel sets up torque forces in the steering, which manifest themselves as attempts by the fork to correct itself, with the result: shimmy, shimmy (speed wobble). Again, the problem is not looseness. Mere tightening fails to correct the problem.
Step 2• After replacing the bearings, the front wheel should be still off the floor. Torque to factory specifications & do not rely on what your buddy says. Notice the bearings are very stiff and need to be “Seated.”• Turn bars lock to lock 25-30 times than re-check the stem nut. It should be looser, indicating that the bearings have squared up and settled into the frame and the “feel” of the steering should be different than when started before turning bars. I am talking from experience on this one. Drove me crazy when I was younger.