I think what you are seeing is the ground speed of the model in a spin rather than the airspeed. The ground speed is the airspeed plus the wind speed over half the turn and the airspeed minus the wind speed for the other half of the turn. if you subtract the wind speed you are left with a constant airspeed and no pulsing.

To help you understand the two points of view consider the following thought experiment. You are in the gondola of a balloon drifting with the wind. You hand launch your model and fly it in perfect circles around the balloon at a constant angle of bank and a constant speed at a constant altitude. An observer on the ground won't see the perfect circles at a constant speed that you are flying. The observer on the ground will see a plane following an epicyclic path with high speed downwind and low speed up wind. To the ground observer the speed of the plane will seem to pulse slowly. Now you, in the balloon, steepen the angle of bank and fly smaller perfect circles around the balloon. The observer on the ground will see the airspeed seem to pulse more quickly. The flat spin you describe is an unbanked very, very tight circle and if you viewed it from the vantage point of a balloon drifting with the wind, I don't think you would observe any pulsing.

Always remember that models fly relative to the air and land or crash relative to the ground.