Like stated above it's not usually the shifter or linkage.A worn input bearing or too much clearance on the mainshaft can cause the gears to move back and forth in the case as well.This is assuming you knew what you were looking at when you stated all the parts looked good.I usually never put a transmission back together without using new sliders and brass.You also need to look closely at the keys on the syncro hub to make sure they are not worn out too.
Gus