The capacitors are mentioned before. But the other problem could be that the lcd isn't able to use the video signal from the old mac card. (Win-)DOS-PCs never used these H-Sync and V-Sync frequencies. There were even a lot of Multisync monitors which were not able to sync to mac frequencies. Try another video card or another flat screen. Video cards built by RasterOps or Radius would be my first choice (running at 1024*768 or 800*600) for testing, or Formac or Villagetronic from Germany. These were popular here.
Which card are you trying? Probably you can identify your cards:
http://www.cls.ne.jp/fujino/Nubus/ (the pictures and the names can be recognized ....
)