Shawn Hixenbaugh

I have 15 sierra 5.3l 6 speed 72,000miles and noticed my rpm needle jumping a couple 100 rpm the coming right back down again and keeps doing so . Does this only in 5th gear. Took to my mechanic he hooked it up to computer and nothing came up wrong. Had him do Trans service couple metal shavings were found in the pan and good amount of fine metal dust. Truck is still acting the same if not worse. Mech recommended transmission replacement. I'm at a loss. Any info would be appreciated. Thanks all