Stone generator generating cobblestone

When making an automatic stone generation by having a piston pull out the stone block from under the lava, whenever the repeater clock is set to either 28 or 29 ticks, I will occasionally get a cobblestone block rather than a stone block. it appears to be that the lava is flowing as soon as the block gets pulled away, so that the water doesn't have time to make the new source
What I expected to happen was...:
Consistent Stone generation

What actually happened was...:
Stone generation with occasional cobblestone generation
Steps to Reproduce:
World file included,
I don't know how precise it is, but here goes.
1, make a stone generation that has the lava flowing all the way, so the last block is the one that makes the stone (I used waterlogged trapdoors as a sort of lava bridge)
2 Make a repeater clock that is either 28 or 29 redstone tick delay
3 have a sticky piston pull the stone out by one block
4 give the clock a ~1 second pulse to start it
5 break the stone as it comes out
6.5 (maybe) if it helps, it is on a Redstone Ready world gen