Engine revving up and down up and down rapidly

Common issues on this vehicle that can cause this.
The first is a broken wire(s) for the power steering pressure sensor. They corrode and break off. The sensor is at the passenger side of the car on the power steering pressure line. The solution is to repair the break which usually requires a new connector pigtail unless you have the tools to replace the terminal ends.

Another issue is a dirty Idle air control valve. Not difficult to remove it and clean it out.

Next would be the EGR valve, the passages and the valve itself have caused issues with idle surge.

Usually the CEL will come on but not always.