Shift from first to second okay. fluid is full. When speed decreases enough goes back into second.
Aug 17, 2019 at 4:56 PM
PATENTED_REPAIR_PRO
✓Experienced Mechanic
1,853 POSTS
Drop the transmission pan and see if there is a lot of debris in the pan, if so it had to come off something and fallen down there, most likely a clutch disc has burned up.
If there is no debris, have the transmission rebuilt or install a new or re-manufactured one even a salvage one.
There is obviously something wrong within the transmission.