Also Tramlining - two distinct issues on a Z3. One too wide a tyre on the front, the car is rear wheel drive so is pushing the front, in this scenario the tyres track the surface of the road, this is proper Tramlining.
Two worn steering and suspension components, in this scenario the wear allows the front wheels to turn (only slightly) away from each other instead of in the same direction, coupled with rough roads this is a very scary driving experience.
Solution to number one, don't go above 225 tyres, use a directional pattern (seems to work best), and if your not into looks stay on 16" wheels, for the forgiveness of the tyres.
Solution to number two - these cars are well over 15 years old, so renew all the bushes, ball joints, top mounts, springs and shocks. Use oem and it will be like it left the factory. If you like it sporty do the same but fit a Bilstein B12 kit and H&R ARB's. (This works really well, personally tried and tested)