Need your help

I have a 94 Cavalier Rs, 171 000km on it.

When I'm driving on the highway doing above 100 km/h the car likes to shake. It's not a constant shake tho which kind of confuses me. I can feel it in the steering wheel. It shakes on for a few seconds, then doesn't shake for a few seconds. Shaaaaaaake.. smoooth... shaaaaaaaake.. smoooth... shaaaaaake.. smooth. It's like the feeling of unbalanced tires.. just its on and off.
Got any ideas?

Thanks

I just finished mine up and have been driving it for 2 weeks now. Mine did the same thing until I put it on the lift! Lower Ball Joint was bad so I replaced it and no more shakey