If you find that large flexi prims are winking in and out or simply remain invisible, then try the steps below. This covers things like trees which use mega prim flexis, for example.
This can also occur with some sculpted (non flexi) grasses.
This should take care of most if not all situations. If not, try increasing the values about slightly (eg, 300 and 5500). Please do NOT set them too high.