I have a top vented gas fireplace (based on the description from this website). Flipped the switch tonight and it didn't come on. Turned the switch on and off a couple of times and no ignition, some times it is slow to ignite. Left it off for about 15 minutes and tried it again...came right on. Burned for about 1/2 hour and the flames went out. Turned it off and left it off. Any ideas where to start?