I just purchased a new 08 Elantra SE. Delivered with ~400 miles, and
I've put ~100 miles on it in the last 10 days. 85% city driving 25-40
mph, and 15% highway on 3 occassions at 55-60.

Driving 3 miles to/from work twice daily (12 miles total each day), on
fully level grade (no hills anywhere), somewhere between 25 and a
little over 30, but even at a constant speed, the transmission wants
to shift up a gear, the RPMs drop a bit, then shortly thereafter it
feels like it's shifting down a gear and RPMs increase a bit, only to
later go back the other way.

It causes a jello like forward/back wiggle feeling. It's like it can't
decide which gear to stay in. It's not noisy or significantly tugging,
just very sloppy and wiggly, and therefore annoying, vs. smooth and
constant.

To rule out my own foot movements, I even tried cruise control on
multiple days at 30, 31 or 29 mph (since it's a safe low traffic and
no stop road for 2 of the miles) and it still exhibits this tendency
to 'slip' up and down right at this speed with no change in grade or
road surface.

I have not noticed this at any other speed, but admittedly haven't
driven much at a constant speed other than 25-30. I did not recall
this feeling in the GLS we test drove before we bought the SE they had
to transfer in, but it's hard to remember what all we felt with the
GLS.

Any suggestions? Will this 'fix' itself based on the learning mode? Is
this a simple software adjustment or is there something mechanically
wrong with my new car? Or is this just the way it works in this model?

I've driven 8 years over the same route in a Cavalier and never felt
this same tugging/release during the trip, and also didn't notice it
for 2 weeks of driving in our 05 V6 Tucson, so it was clearly
noteiceable as soon as I got the Elantra.

Thanks for any input.