Bryant 373LAV error 34

Bacarter

Hello, I started receiving this error in the past couple days. I have removed the flame sensor and cleaned it, same problem existed. I replaced the flame sensor and the system ran for about 30 minutes then shut down with the same error code. I cleaned the green wire connection to ground and checked the connector also. The flame is a crisp blue flame, but will not remain lit longer than 30 minutes and some times just two seconds....
Any ideas would be greatly appreciated!!!!