Dryer's burner will light and burn for several minutes producing hot air. Then after several minutes the burner will cycle off and only produce warm air. Then the burner will again light and burn for several minutes again producing hot air. The dryer takes several hours to dry a load of clothes. Have checked thermostats and all three read continuity on an ohmmeter. The thermal fuse also has continuity and is not blown. Is my problem the gas valve coils or is one of the thermostats opening up prematurely???