89' 420SEL Transmission issue:

[RGROXX]

Hello All, New to the forum... My 1989 420SEL is having a strange trans issue. Driven softly shifts ok, if you drive it a bit hard, sometimes it won't upshift. Almost seems like the tranny is confused if that makes sense? If you stand on it and hit the kickdown, it will slip out of gear totally on the upshift. Putting the car back into park, then back to Drive it will shift ok. Backstory this did suddenly start a while back after being in a passing situation and hitting the kickdown, previous to that the transmission has always shifted perfectly. Any suggestions would be greatly appreciated... I am aware that it could be an issue with the actual kickdown switch itself on the floor... but any other things I should be aware of to check/diagnose?

Thanks so much!!
RG