hi, i had the same problem as you, and these are what i tried, they worked for a while, and every time that the problem seems to occure, i have tried this
and it seem to fix it for a while.
any way -
i changed and flushed my tranny fluid - new filter - gasket, and fluids, (genuine hyundai diamond fluid available at the dealer, it expensive $9 a quart, but it's recomended), and that helped it for a while, and then i discovered when it started to do it again, all i would do it check the fluid level of the tranny while it was hot and running, and it would subdue it's self,
i personnally believe it to have something to do with air getting into the tranny lines somehow, and when i pulled the dipstick out, the air bubbles would clear them self out.
give it a try - that's my 2 cents
