the shift return lever not returning to it's 'static/in-between' position. The problem is especially acute when shifting from third > or fourth gears (up or down).
External: a) Is the shift shaft greased/moving freely at the footpeg bushing?
b) Linkage joints moving freely?
Internal: a) Shift return lever binding under the rear cover where it meshes with the drum gear? Maybe needing grease?