I just purchased a 2006 kia spectra ex and believe I am having transmission prob

What is your question?

While taking off or even driving the transmission will sometimes drive, I could be traveling at 50 miles per hour and all of a sudden it starts dragging and it backs down to below 20 mph, after a couple minutes it kicks back into speed. I believe this is very dangerous and it scares me. I have giving it a synthetic oil change, full tune up and replaced two expensive senors. Nothing has worked and the shop that I deal with said the diagnostis check shows no more codes. I am at my witts end and am not sure what else I can do to fix the problem. It seems like I have put more noney into it then the money I spent at the dealership to buy it. Could you please give me some more options that are not so much of guess work? The check engine light stays on.