@Brad
your explanation may be correct, but it doesn't explain why it happens every other falling zero crossing
i'm thinking its seeing chaos
at some lower frequency, there are no spikes
at the present frequency, the spikes are all the same height
at some larger frequency, the spikes sill alternate between two different heights
etc
real circuits show chaotic behavior, depending on various conditions
it may be that the models of the inductor and the diode produce chaos in a simulation
hence my list of questions in post 3