I've tried searching for this issue but it all seems to just be one gear at a time. This occured over the course of about 10 minutes during a single drive. Moving the shifter forward into first, third, or fifth it pops out of gear as soon as I release the clutch. I have 2nd, 4th, and reverse perfectly fine. I rebuilt the shifter and found one of the bushings is worn. I'm hoping it's just that but this issue popped up over such a short period of time I'm hesitant to think it's any kind of long term wear. Has anyone had this issue before, and is it usually a shifter or transmission issue?

 

Clutch hubs and shift forks are paired as 1-2, 3-4 and 5-R. Since you're only having this trouble with 1,3,5 gears, I'd would say the most likely culprit would be loose or broken engine and/or transmission mounts. As a start, that would be the first place to look.

 

Check your shifter bushing installation since the issue is when moving the lever forward.
If it was an internal shifting issue it's usually in the pairs as posted above.

 

Got it. Inside the extension housing were these two pieces of god knows what. I think one is part of an old bushing. Had to fish these out with needle nose pliers and it shifts perfectly now.

 

the 1/2 shifter return spring is often times the culprit. when it breaks the remnant gets wedged into the shifter assembly and likes to prevent random shift sequences or any at all.