4L80E looses OD sometimes

[Keitha]

In my 1991 Suburban 4x4 3/4ton: I just swaped a 383TBI in place of the stock 350 (had tranny filter and fluid changed at same time). Now there are times when the tranny looses 4th (OD) gear and just goes into neutral. It seems to happen at highway speeds going up a hill, the tranny will downs**t to Drive under load (like it should) then never get OD back. As I crest the hill and start going down, the tranny just goes into neutral. It will go into 3rd when I get on the gas, then go back to neutral when I let off and coast.

It will reset out of this condition when I shut the truck off then restart.

 

Any help would be great.

Thanks,

Keith