Why would my 1997 Nissan maxima's steering wheel shake at highway speeds. I've had my tires balanced and aligned 3 times in a row and it hasn't stopped. It is a rhythmic vibration?

It is obvious that a tire out of balance or alignment is not the problem. Bent wheel, defective tire, loose or worn steering or suspension parts, worn or loose wheel bearing, is your problem. Take it to another place other than where you have been going and have everything inspected.

Check lug nuts and studs.

Check brake rotors if problem is worse when braking.