I don't have a scanner unfortunately and have never used one. I agree with you that a misfire would likely cause a DTC to come up (my usual mechanic said that as well). My misfire I had a while back was caused by a bad spark plug wire and it didn't throw a code either, but it was much less obvious/extreme than what I'm currently experiencing. The dealership said there were no tranny codes and the orig. tranny shop said that as well. The orig. tranny shop DID say, however, that he saw the check engine light come on and go off while he was test driving it up hills, but I've done plenty of hills (since the prob started) and haven't seen it come on once.
I dropped the car off at another tranny shop this morning (they say they provide free diagnosis) and we'll see what they say.
