For about the last 8 weeks, my car will jirk/jolt after diving a little ways

I had the Kia dealership take it out for three separate test drives today while it was plugged into the diagnostic computer device, and no warning lights, no diagnostic codes, nothing came up. They drove it up to 50/55 finally it jirked and jolted , 65/70 mph and below 50/55 mph it drove fine they said... they said it is an internal issue 100% my transmission?? wouldn't something show up on the diagnostic computer device???

This job would be easy if it really did work that way. The idea that the machines tell us what is wrong is a myth. The scan tools are great inventions, but all they are is a tool. With it we can monitor input data and output commands which we can then back up with some appropriate kind of manual testing but that's all they can really do. It takes experience and training to sort through many of the problems that occur on cars by dedicated technicians.

I have not had a tune-up since I bought it (2011) and the used car dealership didn't do one either, so in a nut shell I need a tune-up. I was told that this would be the first thing to have done and would possibly take care of the problem.... and I was also told it could be the coils that need replacing and could be causing this misfire/jolt/jirking...

I am sorry but we cannot be of any more help here as you have been advised what needs to be done!