for it to happen with two different computers might suggest a Samsung problem.
The Samsung forum is here:
I just purchased a 20" Samsung SyncMater P2050 LCD monitor to hook up to my Macbook. I also bought the mini-DVI to VGA adapter, the cabling is right. After powering on the Samsung and booting my MacBook the Samsung is recognized and the snow leopard desktop comes up on the Samsung. I am able to get to the monitor set up in the preferences window, and move the external monitor to where I want it. After a short time the Samsung screen starts pulsating, first at the out side edges of the display, it progressively gets worse, ad then the screen goes black. This happens every time after I shut down the Samsung, and then power it back on.
To troubleshoot I even connected my second Macbook to the Samsung and the same thing happens?
Any advice would be greatly appreciated.