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Does the 24 vac drop out at the gas valve when it shuts down? What are you getting for a flash sequence on the board LED when it shuts down?
You need to check the gas valve voltage, also check the condensate drain on the unit. If it's clogged up the condensate backing up can cause apparently unexplained lockouts.
The voltage dropping out shouldn't affect the gas valve but it will indicate if the problem is with the valve or something else like a clogged condensate drain. These weird intermittent problems are often caused by a condensate back up. Could also be bad connections too. So you could also check all the connections.
This unit has a blocked drain switch, similar in shape to the draft pressure switch and attached to the condensate drain (lower left). If the drain backs up this switch will shut down the unit. A bad board is always a possibility but it's a pricey replacement. Best to eliminate all the other (cheaper) causes first.
Try jumping the drain pressure switch
Connect the 2 leads on the switch together. Did you remove the tubing that connects the switch to the condensate trap? was there any water in it?
There should be water in the trap
See if it it shuts down prematurely
Wait until the inducer fan starts up to jump it