hyundaitech

If you're out of warranty, you may wish to hang a throttle position sensor
and see if all your troubles disappear.

There are also technical service bulletins about reprogramming for poor
acceleration and stumble when cold.

Dustin H via CarKB.com

Thanks for your reply!

It's a leased car, and i've been doing regular oil changes, but haven't done
the other yearly checkups. (So not sure about the waranty)
It's at the end of the lease though, and i'm trying to get this fixed before
i have to return it. (in 15 days)

I will look into a tps sensor, as it's easy and cheap to replace.

Any other thoughts are still welcome

Dustin.

hyundaitech

The warranty is 5 years/60k miles in the U.S. But if it's a lease car
you're returning in a few days, I really wouldn't be that worried about
it. It doesn't sound like something terribly obvious is going on,
especially considering it's been looked at and nothing was found wrong.