Rpms fluctuate when slowing down

My boss asked me about this so I figured I'd post it on here and see if anyone has any ideas. She has a 2004 malibu with the 2.2 L engine in it. Rpms are normal at all times except when she lets off the gas to slow down. When she does let off the gas, it will fluctuate from anywhere from 500 to 1000 rpms quickly. As soon as she speeds up, it stops doing this. Fuel press checked. New plugs and wires. I'm thinking this has to be transmission related because it only does it when slowing down. At idle in park or drive the rpms stay constant. Could the TPS be the problem here? Any ideas?

Failing alternators occasionally do this.

I hope nobody charged her for plug wires. That engine has the ignition modules mounted directly onto the plugs. The electronic throttle control might be acting up. There's no mechanical link from the gas peddle to the throttle body anymore. It's all fly by wire.