The phrase "discutidor de problemas" appears nowhere, as far as I can see, but "discutidor" is a figure that exists specifically in Physics at UNAM, and it is a kind of graduate teaching assistant. The following document refers to this position and to the criteria for appointment:

"Requisitos y Criterios de Asignación de Profesores, Discutidores y Ayudantes de Profesor a los Grupos de las Asignaturas de la Licenciatura en Física [...]
3. Sobre la figura de “Discutidor”
3.1. En la Licenciatura en Física existe la figura de “Discutidor”, la cual comparte obligaciones académicas con la figura de “Profesor”, con la salvedad que es la figura de “Profesor” quien es responsable administrativamente del curso asignado."
http://fisica.fciencias.unam.mx/ACTAS/FISICA_CriteriosAsigna...

This position is listed after "Profesor" and before "Ayudante de Profesor", and this suggests that it ranks below the former and above the latter. "Ayudantes" only have to be undergraduate or graduate students, but "Discutidores" can be "Profesores o Investigadores de tiempo completo de la UNAM" or postdocs, though the position can also be held by a grad student or even an undergrad. So it seems that a "discutidor" is a higher position than an "ayudante".

But both, in American terms, seem to be teaching assistants, who are not solely responsible for a course, like an instructor, but teach on a course for which a "Profesor" is responsible. So I think that we are talking about a teaching assistant, and that "senior" could perhaps be added to distinguish it from an "ayudante", who would be more junior.

This fits with your statement that the person held this position before becoming a graduate instructor, so it looks like a lower position.

In any event, in cases like this where the equivalence is difficult to gauge exactly, I think it's very desirable to leave the original Spanish term and add a translation or explanation in parentheses, as suggested above.