Misfire - help please

hey guys I have a 2011 toyota prius my engine misfires at random times only when idling it shakes and if I put it in drive it either starts shaking or it shakes really low so you don't notice it but it never misfires when I drive it most of the time after a misfire I don't get a check engine light or a code on the scanner but rarely cylinder 2 always shows up and sometimes cylinder 4, I have changed ignition coils, spark plugs, and mass airflow sensor but nothing helped any suggestions what this could be? took it to a mechanic to check it out he drove it and said everything looked fine on his computer I'm not sure what it is and why it only does it when idling but it happens rarely. i'm also only getting 28 mpg

 

282k miles I don't see any coolant leaking but somehow it just disappears out of nowhere