I think you are hitting rocks/ruts/junk on the runway. You have no steering to control it so you spin.

What do you think?

This is from your note above:

If the wind is blowing out, you put your plane down and taxi to the opposite end of the pad, turn and take off toward yourself. I tried to taxi out as far as possible, but the plane is so light, even taxiing with the wind I would go fast enough to loop. I tried to get out as far as possible and head into the wind as much as possible. And I tried to note when the wind was declining or dead. The results were uniform. A short run and a ground loop.

Does the tail ever become airborn? I believe the tail should come up before the wings on a tail dragger. perhaps someone with more experience than I can comment here.

If the tail never comes up, you are not even approaching lift off speed.