It's not normal for your engine speed (RPM) to fluctuate up and down while you're driving, and are keeping a steady footing on the gas pedal. If so, this might indicate that your car has some problems to fix. Most of the time, worn-out spark plugs, vacuum leaks, and a bad idle air control valve would actively interfere with the smooth running of your engine. Thus, causing it to run more roughly and harshly, as shown by the RPMs bouncing up and down. Otherwise, the accelerator pedal position sensor might be faulty, too. Aside from that, dirty fuel injectors and transmission-related concerns may cause this issue, as well.