OS X :: Screen Sharing - Remote Mac Goes All Black (even "in Person") And Needs Power - Down?

I VNC to my iMac all the time (usually from iPhone with Jaadu, sometimes from another Mac with Leopard's Screen Sharing function: Connect to Server vnc://....) But lately it only works for a couple of days. After that, the remote computer's screen goes black and accepts no input. (But I can see/move the mouse cursor.) Once I get home from whatever trip I'm on, I then see that it's the same in person: all-black, no response to clicks or keys. I can move the mouse but I can't do anything at all. There's no solution except to force power-down.

For security, I run the "Computer Name" screensaver with password lock. The first time the black screen appears I see just a flash of the password dialog and then it's gone. After that, I never see anything but black (and the mouse arrow). The remote computer is NOT crashed, because it accepts the VNC connection request and password, and because it keeps churning out work from Folding@Home (which I can see on my stats page). But after a day or two away (maybe one VNC session per day) the screen and input are gone. Anyone else see this? (The only two changes made to that system lately: a new version of EyeTV software--but it's not recording or anything. And the 10.5.7 update. I'm guessing it's something new to do with 10.5.7.)