Sounds like plug wires.  I've replaced a few (but not a lot) of these on
2.4 Sonatas (the same basic car).  Unfortunately, Kia says plug wires are
2/24 only.  But the good news is that you only have two of them.  

Presumably, you should have a check engine lamp if you have a misfire that
bad.  Do you?  Did the dealer present to you that these items were going to
resolve the hesitation, etc.?  If so, I'd ask them what they're going to do
about the $50.  If you did all the things they recommended, used parts that
were not defective, and did the job properly, then you didn't get the $50
diagnosis you paid for.  What you got was the dealer telling you the
required maintenance for 60k.  

To check the wires, remove the plastic cover off the top center of the
engine.  Pull the plug wires off and look at the ends that go down into
the plug holes.  If you see carbon on the outside of the plastic, they're
bad.  If they look ok, put them back on the coils and rest them on top of
the plugs (do not fully clip into place on the plugs).  Then start the
car.  If one of the wires is arcing onto the plug hole wall, you can look
down into the hole as you gradually pull the plug wire up off the plug and
see the fireworks through or off the side of the plug wire.  Sometimes the
plastic just has a small pinhole you can't even see, but it sparks really
well.