If your RPMs are going up and down while parked, it is likely due to an issue with your idle air control valve (IAC). This valve helps regulate the amount of air going into the engine so that it can maintain a smooth idle. If there is a problem with this valve, it will cause your RPMs to fluctuate while parked. It is recommended to have your vehicle inspected and diagnosed by a professional mechanic to determine the underlying cause of this issue.