I'm burning a freestandin Osburn Victorianaire MKII..a fairly large woodstove and the only source of heat for my 1500 sq. ft. home here in Eastern Pennsylvania.I've been burning this stove for about 18 years with only a few repairs to it, it's in decent shape for an oldtimer.The stove is top vented,and the flue is 20 ft. tall, made up of rigid 7" SS flue liner going straight up and out the roof. I've always had a real good draft. The other night at bedtime[10:00pm]I put in three pieces of ash and damped the fire down about 85% for the night [the wood I'm burning is mostly ash, oak and maple and is well seasoned and stored in a dry wood shed]. Through the glass, I could see the firebox quickly fill up with smoke, and I noticed the thermometer on the stovetop read 350-400 degrees. I woke up at 1:30 am to the sound of water hissing from the kettle on the stove. I got up and went into the kitchen and the thermometer read 600-700 degrees and orange flames could be seen rolling violently through the glass. I damped the stove down all the way and the rolling flames disappeared but in a few minutes the flames reappeared as a small explosion of smoke came out where the flue fits into the stove collar...sort of like a bull snorting on a cold winter day. After the small smoke-snorting explosion, the flames disappeared again, only to reappear again in a few minutes with another snort of smoke. I undamped the stove a little bit to about 85% and the flames reappeared and the smoke-norting stopped. Has anyone ever run into this problem? I don't like to see the stove running that hot[600-700 degrees]I feel that's too hot for at night or when no one is home. What causes that smoke to shoot out of the joint at the flue collar?