92 Olds Cutlass supreme 3.1L, difficult shifting

I own a 91 cutlass supreme 3.1L myself currently has 189,000mi which had shifting (it would shift in and out of overdrive at high speeds around 165,000 I had these problems) problems that where somehow related to my fuel system when it wasn't functioning properly. But before I replaced the fuel injection system. It was a first step is to replace the modulator. And I would also check your transmission fluid in case it has been very abused and never changed or if the fluid is dark it could be burnt also.