Brian Lee Rodgers

I have 2003 gmc sierra 4x4 5.7 liter vortex engine with 4L60 transmission. I have a PO751 code. 1st gear and lockout gear works only at first start up. Once driven an few miles I loose 1st and overdrive. I've replaced with a rebuild transmission and problem reoccurring and had that one rebuilt and and replaced PCM. Code and symptoms are back. Any suggestions???