I have a cb 1200i and I recently had to put a new control board in it. My issue is, when I turn the thermostat on the stove will light and only stay running for maybe 20 minutes or so. I do have the board set to the proper setting, I have cleaned the stove, and checked vacuum tube for blockage. I have also checked the cover on the thermocouple. Any ideas on what I should do next?