I was riding around the neighborhood on my System-Six (1st Gen. Rival) and I'm having this weird chain skipping. It happens when I'm NOT shifting.

I'll be pedaling along and all of the sudden it feels like I'm on the lowest possible gear and my feet start over-spinning for about half a second and then it resumes normally.

I'm guessing it's cross chaining (I was running the smallest chainring with the smallest cog). Does my FD need to be adjusted a tad? Or is it the rear one?