Excellent chance it's your Coil Valve. This is like an electro-magnet that controls the valve to supply the gas that supplies the heat. On our dryer, the heat would start and you could feel the vent get hot...but it would not STAY hot. The tech replaced the valve kit (there are two in the kit) for a part cost of $35 and labor cost of $146. You can easily do it yourself (YouTube) and part is here: http://www.amazon.com/Kenmore-Gas-Valve-Coil-279834/dp/B001DEEU8K for a whopping $6 for BOTH. Sadly I looked this up AFTER the tech was done. Fact is I wouldn't have known what to look for, but that's the "ignorance tax" we pay sometimes. Hope this helps someone else. Woulda-Shoulda-Coulda