Usually, "fixing" a friction shifter is nothing more than tightening the D-ring screw. I've never managed to wear one out, but I guess it's possible.Yours do have one part that's suspiciously assymmetric. I've kept the internals of mine lightly oiled, just a slight sheen, and that has worked out fine.
If you want to try something non-destructive, use some blue loctite on the D-ring screw. Assemble, set friction, allow to dry before operating.