doesn't like shifting into overdrive or downshifting

shifts out of overdrive and acts like there isn't any gear then it will finally shift after a few sec. or almost a min. tried servicing transmission helped a little but within a month it has gone back to the same problem. dropped valve body cleaned and installed new gaskets and filter and fluid. lasted about six weeks then started the same thing. getting ready to go back to chevy and end my time with this product