Don't get confused by the previous post - not even I understand it - but here's what you do for avoiding z sorting problems:

- Make sure that you are using the recent engine version. Old versions had a 16 bit z buffer.

- Fix level design faults, like blocks or meshes that are extremely thin or intersect each other.

- Adjust the view clip_near and decrease the clip_far parameter until the requirements of your level is met.

In internet game design forums you might find more advices about proper level design that does not produce z sorting problems.