layers displaying in the wrong order.

i have an issue from time to time where layers display in the wrong order. This is particularly noticeable with 2d layers on a 3d set. Note: the 2d plane is correctly positioned in front of the 3d set and within the clipping plane. It will render properly and if you have the pen tool selected it will appear but with any other tool it will jump back/disappear. A real pain. Don’t know if it is a ram issue, because I’ve worked with heaver sets without this happening.

Any ideas how to go about troubleshooting this?