At 65 mph the centripetal force of the front wheel drives that bike, not your riser height. All things equal...if you had cruise control and let go of the bars at 65 mph the bike should track straight no matter what size risers you have.
A few things, though. If your tracking is to the point where steering comes into play...try the stiffer bushings (I'm assuming they're polyurethane) and see what happens. Installing the stiffer bushings and it NOT being the cure is okay, they're better anyway.
Another factor is body position. Yeah, I know, "Huh?", but I've had brothers that had handling problems at speed and it was all due to their vest flapping in the wind or their jacket being open. Raising the handlebars raises your arms and changes the wind deflection.
Last, your neck bearings. Neck bearings get "cupped" by their rollers over time (false brinneling). In other words...the rollers dig troughs in the race area most used; and that would be the straight-on position. That extra play is enough to create wander and subsequent "wobble" once a harmonic in the steering is set off. Only a disassembly of the front end and an inspection of the neck races and wheel bearings can rule this out. Let us know what you find!
By: 47Knuckle-Dragger on: 6-28-2008