I have a trane gas furnace 11 years old.when the thermostat calls for heat the ignitor will glow but the gas valve opens sometimes and will not open other times.I can turn the thermostat off and on and it will come on sometimes.what is wrong?

What type of furnace do you have? 90% vented with PVC or 80% vented with metal? You may have a pressure switch sticking or a slight restriction in the vent pipe.