How much movement is there in your front axle taper roller bearings?
The manual states a method for arriving at the correct bearing end float to allow for build tolerances and thermal expansion.
This results in wheel that can move a small amount in response to road imperfections, this occurs randomly according to where the tyre contact patch load is highest. On a car with the precise steering of a Europa, this can cause wandering at speed, especially if the nose of the car is high, as yours appears to be from the photograph you posted. Air gets forced under the nose and reduces load on the front suspension and at the same time resistance to wandering reduces as speed increases.
In a corner the bearings are loaded laterally and the movement from the clearance is held consistently in one direction, it therefore steers precisely in the direction the driver is telling it to via the steering wheel.
Check how much bearing clearance you have by jacking the front of the car and trying to move the road wheels against the bearings by grasping the tyre at the top and bottom.
I made spacers to go between my bearings such that the castellated nut could be tightened onto the inner races whilst still maintaining the correct bearing clearance. It made a massive difference to straight line stability and resistance to road imperfections.
