What could this problem be?

Hi, I got a used car a little while ago and it's been great and still is except for this one thing. It's an '03 infiniti I35 and rides really smooth for the most part now when I'm driving at around 30mph, I can feel the car starting to bump around. It's hard to explain but if I were to diagnose the problem I would say that the front or rear left tire is weird and throwing the car off balance every rotation. This issue doesn't happen under or above 30 (ish) mph. Does anyone know what WORST and BEST case scenario is in this situation? Honestly, for all I know something might have been wrong with the road and not the car. Thanks