I started an Avacado tree from a seed a few months ago by using the
toothpick in water method to suspend the Avacado over the water until
it sprouted. Since then I have purchased some potting soil and placed
the pit 3/4 of the way in the soil.

Now it goes through a cycle where it will produce 3 nice large healthy
leaves, then in about a week they start to look wilted and then
starting at the tips back they turn black and die. I then pinch them
off and then the plant produces 3 more nice healthy looking leaves and
the cycle repeats. It has done this 4 or 5 times now and the 'trunk'
has gotten a bit longer.

I am not sure what I am doing wrong and I was hoping that someone here
could tell me how to make this a healthy plant.