Please help. Nobody has had an answer for me.

2001 Deville DTS. 132,129 miles rebuilt motor, 80, xxx on transmission.

In the mornings on both a hot and cold day when I start my car I put it in drive and it doesn't want to move until almost 2000 rpms. Even giving it a second to fully go into gear it still does it. Once it does start to move it takes a while for it to actually wanna go anywhere. I've replaced the range sensor and filters. Had a fluid flush 3 times. Also when driving down the road with shifter in D position will suddenly downshift to either 1st or 2nd gear. Shift indicator on dash will move to the gear it shifted to. What could be causing this problem? I've been to the best transmission shop around my area. Its giving no codes for anything transmission related, they have hooked it up to a laptop and ran computer testing while driving. Only way to get the vehicle back into drive is to either come to a slow roll around 15-20 mph or neutral shut off restart and pit car back in drive and then it will be fine.