aglSwapBuffers & vsync

I wrote a game on a Powerbook G3 with OS 8.6 & Opengl 1.1.2 and it
was running very nice but i switched to
a g4 500 mhz with OS 9.04 and i noticed
a lot of tearing on the image.
The original code of my game installs
a VBL interrupt to get in sync with the vertical retrace but this seems to be ignored on the G4 , also recompiled the game on a Graphite Imac with OS 9.0 and
the problem still exists .
Later I wrote a llitle program to test the same VBL interrupt but with some animation done with QuickDraw and gWorlds and there isnĀ“t a trace of tearing so i think maybe the problem is the ATI RAge 128 driver or OpenGL on OS9 .
Does anyone knows something about this?