Most highways are slightly banked. The travel lane banks to the right, speed lane to the left. It is not uncommon for a car to steer slightly in the banked direction. BTW - keep your hands on the wheel at 100 mph. Or better yet - drive a safer speed before you kill someone.
At 100mph you've got a lot of aero effects that are confounding the situation. Taking your foot off the gas will slow the car down dramatically (as compared with taking your foot of the gas at 60mph) so there's going to be some weight transfer and thus the suspension is moving around.
If the road has enough crown in it that it's affecting the weight transfer of the car left and right, I could see that there could be some "torque steer" effect. Does it make a difference if you are in the right or left lane?
I bought a '99 Grand Marquis and thought it had an alignment issue. It was slop in the recirculating ball steering + the crown in the road playing tricks on the steering. It would pull in either direction depending on road conditions.
The slight twitch of the wheel from a stand still appears to be normal trait of this car. I drove the service mgr.'s car at the dealership and his also does the samething. I have seen other posts with the issue mentioned as well. Recently I have driven a Ford Escape and a Freestyle and they also had a similar twitch. Like you said, not strong, and if you keep you hand on the wheel yiounever notice it.
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