The boat steers a zig-zag course (oversteering)

Adjust the windvane tilt
If there is no internal friction it may help to increase the angle of the windvane axix from 20 dgs (leading edge of the windvane blade parallel to the mast) to high angles (leading edge of windvane at 10-15 dgs to vane mast). This may help especially on courses with the wind between 90 degrees (side wind) and 180 degrees running course.
When you increase the angle, the travel gradually decreases.

Connection chain/tiller
It may also be the case that the boat moving down waves causes a change in direction that can not be corrected quickly enough.
This may be solved by moving the chain hook closer to the boats rudder stock/hinges. See "pendulum rudder stays at the surface" for more details.

Internal friction
Check if the windvane moves freely and smoothly. If it jerks there is probably friction in the path between the windvane and the vertical rotation of the pendulum rudder. Check the next page.
If there is no friction in the system, also check the rudder arrangement for friction too.