hard shifting when trans heats up?

[2002 grand prix gt]

i have a 02 gt and when i first start driving the tranny shifts fine but once i drive around for 45 minutes to an hour it shifts hard thru all gears? i read somewhere that it had something to do with the flex plate. it does slip into 2nd gear would a shift kit help out the slipping going into 2nd? also im oredering a temp gauge soon to see if its running hot.

 

[dsmuts]

Do a fluid and filter change and add a bottle of lucas stop slip and that most likely will take care of it.

 

[Bronco Boy]

Could possibly be code P1811. Get a scan tool and check for engine codes.

Go here for my write-up.

Been almost a year since I installed the shift kit. Still going strong, runs as tight as can be expected and hasn't gotten any worse with time. She will shudder on the 1-2 shift when I romp on 'er, but I don't romp on 'er to much these days.