Now we know it's not your overdrive, but it can still be your torque converter lockup solenoid. What it does, is when the trans oil and engine coolant temperature are hot enough, it will send a signal to the tcm (transmission control module) to say to the TCC solenoid to activate. This will lock the transmission in gear . What you are experiencing , in my opinion, is called torque converter shudder.
This is to save gas and wear on the trans. Now confirm if i'm right, when this happens and you press on the gas (in order to force a downshift), the shudder will go away, right ? Then when you release the gas, it will start again..?
Also, it should not be doing it right after you start( because the oil and engine coolant are still cold, therefore not sending the signal)..
There is one easy thing that you can do to see if it is. It will require you or your mechanic to disconnect the TCC Solenoid valve connector. Unfortunately, this connector is under the transmission oil pan. So you will need to , drain the trans oil, remove the 10 or so bolts holding it, then disconect the tcc valve connector and re-assemble everything and put new oil.
It may seem hard , but really it is not.
download here the transmission repair manual for your pathy, that I have put here for you : http://www.4shared.com/file/242412000/e ... nline.html
and look at the page AT-257 to get a look of what the TCC valve looks like.
When you are going to remove the trans cover, you will see two connectors, one for the TCC and the other for the oil temp sensor. Using AT-257, find and disconnect the TCC connector.
Put back the cover and oil and run like this for a day or 2 ... you'll know for sure after that. You see, the TCC receives the signal from the TCM and this thing receives signal from the vehicle speed sensor and throttle position sensor... Having the TCC disconnect will not change anything to your normal driving . It will only cost a tad more in gas... but it's for a day or 2 so , it should not be dramatic.
hang in there, we'll find it...