8 -3/4 Clunker Question?

My 489 8 3/4 rearend (4:10 Richmond Gears)that is supposed to be a fresh rebuild has issues. It seems to be ok on short runs (when cold) but after 25 miles or so of continuous use (Highway Run)it appears to actually have too much backlash and the gears start clunking when I slow down. When the car cools down seems to be ok again. Any rear end gurus out there got an idea what is causing this. I'm going to get it out this week and get it checked.

might want to check pinion nut. needs to be 240 ft.lb. if i remember right.dont ask how i know.