My Lennox gas furnace runs for 1 to 2 hours, then shuts down
Some of the electronic ignition modules used in older Lennox furnaces fail by becoming weak or intermittent and after several failed retries on a single start cycle the furnace remains in a no-heat condition until the call for heat is removed and reconnected.

If the failure only occurs during a normal start cycle, and once lit the flame does not go out before the room reaches the thermostat set point, then next time a failure occurs test for 24vac across the power supply leads on the electronic ignition module.  If it has power but there is no spark then you probably need a new module.

EDIT:

One word of caution.  It would be best to witness the ignition unit failing before ordering a replacement.  Rarely a gas valve can intermittently stick closed and cause similar symptoms.