So sometimes while I'm driving down the highway the car (which has done 210 000 km) will start to violently vibrate. Only really small movements and I can only feel it through the steering column.
You probably have two front tires, each of which is slightly out of balance. Or possibly one or the other has a slightly bent rim. Or, some combination of the above.
I keep saying 'slight' because this problem only occurs when both tires are in a certain relative position to each other. The tires probably have a slightly different rolling circumference - perhaps .2mm or .3mm difference. So, you drive down the road and every few miles it happens that the two tires are vibrating in sympathy. After a few more miles, the problem goes away, not to return until that specific alignment occurs again. If the road is not perfectly straight, the curves may delay (or accelerate) the occurrence of this problem.
No easy fix. Try adding a bit more air to one front tire or the other. You could try rotating the tires on one side of the vehicle front to rear. Or, rotate all tires front-to-rear. See if that the helps. It may not solve the problem but you won't notice it so much if the problem is on the rear.
With the tires unmounted, check the rims carefully for bends. Also check them for run-out - lateral and radial. This is frequently an issue on boat trailers, the manufacturers of which use the cheapest, crappiest rims known to mankind. They always get bent, out of round, etc.
With the tires unmounted, inspect the tires for a slipped or broken belt - sometimes it is visible as a bump or lump you can feel on the inside of the tire. If a tire has more than one patch, you might move it to the rear. Make sure you have no foreign objects inside your tires (rocks, bullets, etc.)
Have your tires balanced. Pay particular attention if a tire requires a lot of weight to balance. Also, note if a tire requires a large amount of weight on the inside or outside. Good quality tires in good condition should balance without the addition of much weight.
Basically, attention to details will solve the problem, but it may take a fair amount of effort.