clincoln007, my first car ( a 1969 Volvo Amazon sedan ) had an electrical issue, which I believe is of exactly the same nature than the one causing your auger to modulate your convection blower. When the indicator lights were turned on ( no matter what direction ) the red tail light would get dimmed every time the indicator light was blinking. The reason for this was simple: A bad ground connection to the chassis would limit the current and a voltage drop would build up over the bad connection, whenever the indicator light was blinking.
The most likely explanation to your "modulated blowers" would be a badly worn low limit switch, since this carries neutral to the entire stove, when stove has passed the warm up procedure. A worn low limit switch will act like a current limiting resistor. A voltage drop will rise over the low limit switch, when the load increases. This will make the switch hot, and as it gets hotter, the resistance will increase along with the voltage drop. Eventually the convection blower could almost stall.
If you have an ac-voltmeter, and some experience in measuring on equipment carrying line voltage, a simple measurement of the voltage drop across the low limit switch will tell us exactly what happens when the auger is "modulating" the convection blower: If the voltage drop increases significantly, when the auger is on, the low limit switch has to be faulty.
Take care, 120 Volt/60 Hz can be lethal!
The simple logical sequence of aforementioned warm up procedure can help us further to determine that the low limit switch is the culprit. You see, the low limit switch is bypassed by an on board relay ( controlled by a one shot 30 min. timer ) allowing the stove to warm up.
The low limit switch is also called a POF switch ( Proof Of Fire switch )
So one simple question: Does the convection blower run ok in the first 30 minutes? ( Stove must be all cold, when you make this test ). If it does, the low limit switch is most likely due to replacement.
Oxydation in the Molex plug, where the main supply ( both phase and neutral ) is connected to the control board, could also cause this issue. Unplugging/plugging the Molex plug a handful of times is often enough to clean the contacting surfaces of the metal. If you have some contact cleaner to spray inside the Molex, this will help deoxidizing the metal pins in the plug.
Good luck and take care. Keep us posted.