Problems with my nitro

I purchased my Nitro in December and love it, except for the tranny. I have had it back to the dealer 4 times. I have assisted the dealer with the info I learned on this site. First 3 times they put it on the computer and rebooted it . The last time they replaced the torque converter. It is better but still has a hesitation at 40 mph. What would be the next step? What have others done to correct this problem? Shouldn't I expect a brand new car to run better than the 4 year old car I traded? HELP