Circle Misalignment on only 50% of depth

Hello, I’m not sure if this has happened before, I’ve searched and searched but only keep finding circles not being good circles. The issue I am having is I get a perfect circle for the first 50% of passes, and then the next 50% become misaligned, creating a perfect circle on the top half and an oval on the bottom half. I’ll try to post a picture tonight when I get home to make it more clear

The second it starts to carve messed up check the V wheels. They like to do whatever they want to do. It could also be pressure on the bit if you are cutting out circles causing loss of steps. If that were the case you need to check your belts and pully screws, and that the V wheels are tightened.