Furnace is around 20 years old. Sometimes the main burners light and sometimes they don't. When they don't, I go downstairs, turn the blower motor off and flip the black switch to 'off' on the little beige box (smart valve?). Wait 30 seconds, reverse the process and the main burners come on. They may come on the next few times but when it starts to get cold in the house, I go downstairs and see the main burners haven't fired up. Doesn't appear the plastic hoses are clogged. Any suggestions where to start looking?