Q: Why does my car's RPMs spike randomly at a constant speed?

Depending on the terrain where you live, this may be a normal function of the engine downshifting. The vehicle should accelerate slightly when this happens, or should up under an increased load of some kind. With the age of your vehicle, if you have never had your transmission fluid changed, this would be an appropriate time to have it serviced. A clogged transmission fluid filter can cause similar symptoms, and this is a standard replacement with a transmission service.

Mechanical issues with the transmission can also cause an engine surge. However, always start with the simple things first. If the transmission checks out, you will want to turn your attention to the fuel and spark systems. EGR valves, IAC valves, restricted exhaust systems, and poor air intake can also cause similar issues. Start with the transmission, and then work your way down the list until the problem is solved, so you know all the components listed are in good condition.