I noticed a few weeks ago that sometimes if I am coasting down a hill, the car surges a little bit. It did it again this morning. You can barely feel and hear the car gaining a couple of hundred RPM while off the gas and coasting downhill. It doesn't do it the whole time. And it eventually stops after doing this a few times. Anyone have any idea what could be causing this? And should I be alarmed for a possible worse problem?