SUburban NT32 furnace cuts off after 30 minutes

Agree with limit switch or board- limit switch is the most common failure in that model, but the boards are known to fry, because of where they are mounted.
If you do have to replace the board, I would highly recommend the Dinosaur extend-a-life kit (along with a Dino replacement board).

You can do a very rough test of the limit switch by pushing on the metal element in the center of it- snap it a couple of times. If the flame comes back, you have found the problem.