Clutch pops out of gear from 2nd to 3rd

[crazyazcat]

New to motorcycles in general but have a good understanding of mechanics so please bear with me here.

I have a 1981 XS650 which pops out of gear when shifting from 2nd to 3rd. I can drop it back into 3rd afterwards but it consistently pops out going up. Any ideas what I can check?

Thanks,

-=Rob=-