I'll keep this thread alive.
In the process of doing an internal VGA mod for my Dreamcast. I was about to wrap it up today, but when I use it on my monitor, it works for a short 10 seconds. Power cycling doesn't bring it back. Removing the plug, draining the power by clicking the power button a few times, and replugging it back in gets it to work, but again, only for 10 second.
The only thing I know that MIGHT be causing the problem is the VSync off the board. While soldering the wire to the point, I accidentally pulled the point off, too!
I managed to find a small point just below the VSync spot that I soldered it to. Think that might be the cause?