Q: Engine shakes intermittently after warmed up with car in "drive" at a stop. Single shakes at a time.

Hello,

My instructor at the technical school sprayed my throttle plate (butterfly) with cleaning fluids to clean plate and throttle body. There was just some carbon buildup inside.

Later we (students only) changed the spark plugs with NGKs that came originally on the car. No torque wrenches were used. Coil packs for cylinders 4 and 6 had been incorrectly connected and led to P0300. We corrected the problem. Engine ran for a minute or so on 3 or 4 different engine starts with the issue before we swapped connectors.

A couple days later I notice that while my car is stopped while in drive, the car engine does a single shake. It happens again the following day. Both times were after I had taken short trips and warmed up the engine. Just yesterday I took the car on a long trip and every time I came to a stop, the single shakes occurred often. So it definitely happens after engine is warmed up.

Runs fine on the highway. I even get the same fuel economy as before.

Any ideas?

Thanks!

My car has 149600 miles.
My car has an automatic transmission.