Originally Posted by
churnman
Sometimes a sign of seat too high. Rocking pelvis.
Bouncing at high rpm can be seat too low, not too high.
Rocking pelvis can be seat too high (reaching for the pedals).
These are just starting places however, useful only in the absence of more information.
But for the OP, try raising the seat 5mm at a time until the bouncing calms down. if you get hips rocking, go back the other way.