I have run many tanks of gas through the car with the problem. A buddy who races dirt bikes said California has been putting in more ethanol and that may be it. I tried blending super unleaded with 100 octane unleaded. I think I picked up a few horsepower but didn't fix the idle problem. The race gas here is over $9 a gallon, ouch. Don't know if it threw any codes.
I used to build race engines over 20 years ago. My normal routine with something like this is change the spark plugs, clean the throttle bodies. Had a similar issue with my first Toyota Tundra, an independent said I needed a throttle position sensor, I cleaned everything out, new spark plugs and it was perfect.
The problem with dealership mechanics is flat rate, they get paid x hours for a job so there is incentive to rush it. If I have them clean the throttle bodies they will spray carb cleaner in with the motor running. That washes down the cylinder walls with the crap which isn't good for the engine.
If I want it done right I think I have to do it myself.