thanks for all the responses so far.
The power steering pump and hoses are fine also, there is no noises, no whining etc. not even when reaching the normal lockpoints. Steering itself is normal, no excessive power needed. Have compared it to other cars of the same age and it feels exactly the same.
The weird thing for me is that the problem only happens when driving forward and only under load, i.e. it doesn't happen if the car is on the lift.
I'd love to know what causes the steering to lock, there must be something catching the wheel, steering components etc. but what?