Q: Car is idling up and down while either parked or when driving.

I recently replaced my Started in my vehicle. Since I got it back on all of a sudden its idle was off. I took it to Oriely's and had them put it on their computer. It came out with 4 error coded. It ended up being a sensor called oem or something like that I paid for the sensor and replaced it while I was there. It honestly went from its rough idle and it going up and down in rpm's randomly to perfect. There was no noise whatsoever I could barely hear the engine we thought it was fixed so he reset my computer check engine and let me go home. I drove the car and it was perfect but a few hours later it started acting up again. I took it the the local chevy dealer and they said that they put it on a computer and there was no issues. they said it was due to Carbon build-up, so I paid 200 and some dollars to get the engine cleaned. Guess what it did nothing other than it not idleling quite as low when the ac kicked on. essentially nothing has got fixed what is the issue?

My car has 160000 miles.
My car has an automatic transmission.