The problem seems to the the Taco SR503 switching relay and not the boiler. You need a hire a competent tech to diagnose the problem and make the required repairs. You may need to replace the switching relay.

The flame rollout is a sign of a blockage or obstruction in the flue. If not a bird's nest or chimney debris buildup, there may be years worth of soot where the boiler vent attaches to it. First, check for direct venting visually. For peace of mind, if for no other reason. Traditional valleypacific.ca valve with a standing pilot and a variable rate gas control downstream of it that controls the burner flame by sensing the water temperature through the bulb and cap tube. In the language of current rules, refer to it as an analog "water temperature reset."