Delete Every Other Row Until

I have a .csv with tons of data, and I need to delete every third row to get the total number of rows down to 32000 - so I can plot them. Up until recently I was using the following code to do this:

lastrow = Cells(Cells.Rows.Count, "A").End(xlUp).Row()

For z = 2 To lastrow Step 2

Rows(z).Delete shift:=xlUp

Next z

In order to save time I'd like to incorporate Do Until somehow, so that when the lastrow = 32000, the deletion process stops. I tried the following, to no avail (it ran, but didn't stop at 32000):

lastrow = Cells(Cells.Rows.Count, "A").End(xlUp).Row()
Do Until lastrow = 32000

For z = 2 To lastrow Step 2

Rows(z).Delete shift:=xlUp

Next z

Exit Do

Loop

For some reason I'm not figuring this one out.