What could be a problem with DSI Suburban hot water tank?

Keep in mind the current path for the flame sensing- it has to go through the spark wire and electrode, then through the burner, attachment to the case, etc. before getting back to the circuit board via its ground connection.
My take- make sure all of the burner mounting points are tight- I even run ground wires from the electrode mount back to the gas valve, or closest good grounding point.