94 F150 XLT Transmission - hard shifting

Howdy all. The automatic transmission appears to shift hard after sitting a while in the rolling parking lot AKA commute time at slow speed (5 -10MPH). I
sometimes shift from Drive to 2nd and that seems to help it. Once up to normal speed (drive), I do not detect any problems but if I slow down to stop or turn the corner, it will again shift hard.
Problem really surfaces during the summer while not so much during the winter probably due to viscosity...
I've changed the transmission fluid but this did not seem to alleviate the problem.
Appreciate your suggestions/comments before I look to take it to a local garage.
Thank you, Rey
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