My car feels like its going over small bumps in the road (on a perfectly smooth road) while under 20mph?

when I go faster than 20mph the "hopping" feeling stops. At about 60mph the car shutters/vibrates for about 2 seconds, and then is fine for about 3 seconds, then vibrates again. This goes on for the entire time I am traveling. One of my friends told me it could be a loose lug nut on a tire, allowing the one wheel to flex slightly while in motion, or a clip weight fell off and is making a tire out of balance. Does anybody else know what could be wrong? (Its a '95 ford escort LX)