ok truck sat for several years because it wouldnt shift period. previous owner thought the worst.
once i got it home i changed the trans speed sensor and the truck began to shift. now here is where it gets weird....

sometimes it will shift fine when cold when taking off slowly. any other time when cold it attempts to shift to third but will shift to nothing, just free revs.

90% of the time it will shift by letting off the throttle as it shifts to third with OD on or off

the other 10% of the time if the first trick doesnt work i have to accelerate to 35 and manually shift to D with OD off and let off throttle untill i feel the truck faintly bump the speed down indicating im in 3rd n not neutral.

not sure where to go from here. going to do the trans filter soon. BUT id like some direction as to where to go and what to check while im down there. also contemplating removing and cleaning the valve bODY. (I CANNOT FIND ANY INFO ON CHECK BALL LOCATIONS BETWEEN TRANS/SEPARATOR PLATE/VALVE BODY ITSELF)

BTW truck has no codes coming up. ecu thinks everything is cooperating

EDIT** When talking about shifting in general here i mean the third gear only. as it is the only one im having issue with. second rarely shifts late but thats it. all other gears and OD function as they should