I'm having a similar issue with graphics disappearing, too.
I'm currently running Landmark on a new Mac (MBP 15" 2.8 Ghz 4G ram). The problem I'm having is when I start to put plants on the plan and then zoom in or out the graphics disappear...However, when you drag the cursor over the plants the outline shows, but nothing else. The plant images don't reappear until you zoom out or in, and even that doesn't always work right away.
It's beginning to become very frustrating, especially with larger drawings.
Any suggestions would be appreciated
Thanks,