[Too Many Projects]

I suggest inspecting the bushings on the ends of the 3-4 rod for any wear or abnormality. Check the fastener at the shift shaft for any wear or looseness that allows the arm to move on the shaft. Also, that the adjustment is still correct. Components seat in and can be off enough that the 3rd gear slider may not be traveling far enough to fully engage the keys in the hub in the detent in the slider to help keep it engaged. Do the simple stuff first before looking at an internal cause.