[Blue Bowtie]

It could be the torque converter clutch not releasing cleanly, caused by a sticky solenoid valve or spool. Do all the ranges seem to operate normally when up-shifting?

When driving on level ground at speed, maintain speed (cruise control off) while lightly touching the brake pedal to see it the TCC releases. You should be able to force the TCC to release. Remove your foot from the brake and the TCC should engage within two seconds. You should be able to do this several times to see how the TCC applies and releases.

Given the age, and with the possibility of considerable mileage, if the transmission oil and filter have not been changed recently, it may be due.