The fluid level should be checked with the motor warm, running at idle and gear select in PARK.
HOWEVER, I would give it a check with the motor off also.......the fluid level should be higher with the motor not running, and drop down some when the motor is running.
If this is not the case......it could be a pump problem.
Do you have a Check Engine Light lit?........or a flashing O/D light in the gear select lever?
Once you verify the fluid level.....the next thing that I would do, when this happens, is to select "1" or "2" instead of "D" and see if that helps.
Another thing that can happen is a defective Transmission Range Sensor (TRS).....AKA Neutral Safety Switch.
This part provides input to the PCM (computer) as to what gear you have selected.
A test that one can do........with the motor running......unplug the electrical connection to the TRS, and see if it solves the problem.
From what I have read, the shifts will be more abrupt, but it will shift.....providing that the problem was caused by the TRS.
You have to start the motor before unplugging the TRS because of the other function that it serves.......neutral safety.....preventing the motor from starting in any gear but PARK or NEUTRAL.
IF the fluid has not been changed for over 30K miles, it is due.
The correct fluid is a fluid that meets the Mercon V specification.