burners turn on but as soon as the blower kicks on (about 2-3 min. after flames start), the burners go off, the fan keeps going for about 30-45 seconds then turns off --- this repeats every 6-8 min. --- I thought it was the thermostat so I installed a new one (programable) but furnace continues as before --- the thermostat is set at 70 but the temperature never goes up past 64-65 --- thanks for any help