If the vehicle has an intermittent misfire, low fuel pressure or a vacuum leak it will not always set a code. Many times it takes and advanced scanner with some tests and watching some live data to catch a problem like this. Please let me know what engine( DOHC or OHV) you have and how many miles. There are a few things we can check, just need to know what engine. To diagnose over the internet many times we need to go back and forth to narrow down the possible causes. If the mechanic used a low function scanner he might miss what we need to see.
lets start here
My engine is a DOHC with 83,000 miles
These engines are known to have the PCV valve hose fail and leak. Vacuum leaks can cause irregular idle. These also have COP ignition systems, coils over each spark plug. The coils can fail , if intermittent enough they will not set codes. For both of these issues and advanced scanner with live data can catch these.
here is what the hose looks like, under the throttle body.