Could have been torque converter lock up acting up
Torque converter is a "clutch" of sorts, the computer has the option of releasing or locking it
When you are stopping computer needs to release it or engine would stall, but when driving you get better performance and MPG when it is locked.
Since you just change the trans fluid you could have got some air pumped in to torque converter, air can get trapped in trans cooler or upper trans areas, it will get cleared out over time.
If torque converter can't fully lock, it will shutter as it slips, same as clutch can shutter if it slips and grips.
Since there were no codes and all seems well now, there is not much you can do but guess at possible problem.
Which is what the above is, a guess.
You can use either spark plug, the "a" usually denotes Suppressor type spark plugs, if you are getting Radio noise from spark system then use "a" model, if not then regular is fine and usually less money
Last edited by RonD; 01-03-2017 at 11:31 AM.