Incredible and strange... I solved the problem! I changed the nubus slot at the card! From the lower to the upper... Now supervideo work and my lcd don't lose the sync! 

For the graphics performance I run Macbench 3.0 and the graphics score is 62% of Powermac 6100! Slow? I run the test at 1024x768x8, powermac 6100 run the test at 640x480x8!!!! And in some test I have exceeded the powermac!

Is good? (i not have q700 onboard video results)