I just ran a few checks on the F-111.
All tests done on autopilot , at speed 2.52 Mach.
(1) at 45,000' Elevator trim varied rapidly in range 0.2 to 0.5 , velocity vector bouncing above/below horizon bar.
(2) at 35,000' Elevator trim varied rapidly in range 0.0 to 0.3 , slightly reduced bouncing.
(3) at 25,000' Elevator trim solid on 0.1 , rock solid , no bouncing .
(4) at 15,000' Elevator trim solid on 0.0 , rock solid , no bouncing .
In this particular aircraft the higher altitudes resulted in bouncing or shaking .
Cheers
Karol