Today when driving around a gym parking lot I noticed unusual vibrations through the steering wheel. I stopped and let the engine idle. It stays at around 800rpm a few seconds then slowly goes down to 500rpm. At that point I notice the vibrations and RPM jumps back to 800. I went into the gym and then came back later but couldn't reproduce this issue again. This hasn't happened ever before. Is this something serious? I am worried because I need to go on a 200 mile trip soon this weekend.