I have a Bryant Plus-80 down draft natural gas furnace. It seems like it is not starting reliably. If I pull the power plug, it will start up when I plug it back in and run normally. However, a few hours later, it is flashing warning or error codes and not running.

The error codes I have seen are (from the front panel label):
14 - IGNITION LOCKOUT - Control will reset after three hours. Refer to #34.
21 - GAS HEATING LOCKOUT - Control will not auto-reset. Check for stuck gas valve relay on control or miswire to gas valve circuit.
34 - IGNITION PROVING FAILURE - Control will try three more times before a lockout #14 occurs. If flame signal lost after trial for ignition period, blower will come on for 90 second recycle delay. Check for
-Oxide build up on flame sensor (clean with fine sandpaper)
-Proper flame sense microamps (.5uA DC minimum)
-Gas vale turned off - Manual shut-off valve - Low inlet pressure
-Green wire must be connected to furnace sheet metal
-Inadequate flame carryover or rough ignition

Furnace has a White-Rodgers 36E gas valve.

I have cleaned the furnace and the filter is only a few weeks old. House is about 16 years old, no work has been done on the furnace recently (or at all that I know of, I am not the original owner). Upstairs furnace right next to it works fine, so I don't have any concerns about gas pressure.

What should I do first?