MAke sure the kick down switches aren't stuck\jammed by a floor mat.

X2 on the fluid change and a bottle of trans-X\Gunk trans-medic (whichever you can source) and see if the issue goes away. Could just be a stuck gravity check valve. I would avoid the lucus product initially. It's a thick honey like fluid designed to work on transmissions with worn out pistons and leaky valves to allow them to build up more internal pressure.

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2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold