Remove the shifter cover.
Inspect the shifter shaft(#14) for wear.
Inspect the rivets(#22 or 31) that retain the shifter Yoke. Make sure they're not loose.
Inspect the Yoke(#34 or 21) itself for excessive wear.
If Yoke is tight, inspect the slot(in the sliding gear) that the Yoke fits into for wear. If so, the sliding gear will need to be replaced.