1995 Cadillac EL Dorado Transmission Slipping

After running car for 30 to 45 minutes at various speeds (30 to 65) all is fine and THEN-- when going from 40 to 65 miles per hour the motor continues to race at 4,000 to 4,500 rpm's. It will not settle down to a normal drive rpm of about 1,500 to 2,000.
Is there an additive that may help.
A few weeks ago dropped trasmission pan replaced both filters and put new fluid in.
Looking forward to your response.
Thanks
Ron