SUburban NT32 furnace cuts off after 30 minutes

The limit switch should cut back in if it were over heating- but the contacts in it get oxidized and cut the voltage down below what is needed by the board.

Another item it could be is thermostat, if it is just for the furnace. Those contacts can get oxidized as well, so when it get close to the set temp, the pressure on the contacts gets lower, again cutting the voltage- the fan relay will close with 10 volts, running the blower, but the circuit board needs 11 at least.