P0303 code and shops not able to fix it apparently.

What is your question?

When the car is in idle it runs smooth without any notable changes in rpm and no revving. When it reaches 45-55 mph i feel a stutter kicking in with a steady rhythm. ran a OBD code on it and a freeze frame or mode 6 with it noting it wouldnt pass the emission. I have replaced the spark plugs and gapped them @.044. i have gotten a new ignition coil and swapped the coil. i have swapped the coil with the other coils and still registered the code. no other code of p0300. It use to have a code of P16D0 and i replaced the intake solenoid valve and that code cleared up (both registered around the same time). I changed the Spark plugs roughly 4 months ago and coil around three months ago. Me and my SO are military so we havent had a lot of time to take it in. We took it in to the local shop and the most they performed was an oil change to fix the p16do code (didnt work and lost 250 in the process). The only recommendation they gave us was to take it to a chevy dealership. thats 50 minutes away

What makes this problem better or worse?

45-55mph

How long has your 2014 Chevrolet Malibu had this problem?

4 months