How can I fix my Hyundai Sonata 2003 from stumbling?

Please help...I have a Hyundai Sonata 2003 65,000miles, bought at 50,000miles in Oct 2008. the problem is that lately it kind of jerks while driving. its not consistent. sometimes it jerks/stumbles at 80mph, sometimes while idling at a stop light and mostly not at all. we had it looked by a professional transmission shop and they said it was all OK....
can there be anything wrong or am i being paranoid?
Also what is the thing called that you can add while filling gas in the tank so that it kind of cleans the insides? will adding that be a good idea?