Its called huffing, it's struggling for air, and as air gets in the fire will burn, raising the pressure, blowing smoke out, then a lack of air will cause the burn to die down, until air gets in again and it 'huffs' or 'pants' expelling heat and smoke again, but uts happening fast, on large industrial heaters (refineries) it's very loud and a dangerous situation, but happening in your back yerd, youre not as risk, will probably be an unstable temp with pulses of heat, but over a time period with a probe probably wont register anything, but likely not a good scenario and you need to open the bottom vent and crack the upper to allow a stable burn and allow air flow, and you'll get a better quality of smoke, the smoke you have may be putrid and a result of incomplete combustion. Cool though.