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Either board is not sensing flame or you have a gas pressure issue.Do you have a volt meter to see if we are losing 24v to the gas valve as soon at instant burners go off?
Lets do this/Turn to call for heat and allow burners to ignite.Put leads on gas valve terminals,there will be 24 volts. Then very second burners go out watch and see if we lose the 24v or if the 24 volts stays a couple of seconds until flame sensor realizes flame is gone.