4T65E Transmission Question

The trans is a 3 speed w/ od. So what is the difference between driving in 3rd and driving in " D ". For some strange reason I get a slight "bump" like a harsh shift same place every day at the same time. The car starts out in first, after about 5 or 6 feet, the bump. I keep going the car shifts to second later ok. Think it might be the PCS, not sure. Mostly does this starting out in the morning, has done it other times very rarly from a stop after driving ahile. If I take the selector out of "D" first thing in the morning and put it in 3rd all is ok....any ideas ?

Just a matter of semantics....you have a four speed transmission....over drive is fourth gear....

Check fluid level and condition?

Scan the tranny for codes?

just did fluid and filter on the trans did not help. yeah, well outside of one uses OD the other does not.. stuff like torque converter lock up, PCS , was wondering if in 3rd and not OD the trans relies as much on the PCM and the MAP and MAF...ETC.