- Automatic gear "hunting" is pretty common.
The "shudder" you mention isn't.
The engine lugging is creating some additional strain which likely makes the shudder show up.
I'd look for a slightly loose mount on the engine or transmission...
For me, the gear hunt only happens on steep hills where my speed is just below 75 (the speed limit here).
Therefore the options are drive a bit slower, drive a bit faster, or use the sport mode on the shifter to manually select a gear.
In your case, I am guessing that you speed up and then have throttled back about the time the transmission is shifting.
If at about that same time, you start going up a hill, the transmission will be in too high of a gear for your speed, and it would (should) downshift on its own.
By hitting the throttle harder you are forcing that to happen.
Pretty normal except for the shudder, IMHO.