The fact that they found no fault codes doesn't surprise me. Did they check the date code on the radiator? Also, how is it that they couldn't duplicate the problem, while it occurs every time you drive it? The fact that the symptoms have improved since the transmission service was done tell me that the lock-up clutches in the torque converter are the cause of the problem. Go back and ask them to dis-able the torque converter lock-up feature (this can be done with SDS), and drive the car to see if the problem goes away. I'm betting it will. The lock-up feature will re-set itself at the next key cycle. If the torque converter is found to be the problem, and there’s no visible coolant contamination, the dealer can still check for trace amounts of coolant in the ATF.