You really shouldn't have any problems if you start out with standard VESA modes and go from there. Start with 640x480, 60Hz, 31kHz and 8-bit.
Also, have you tried it with the latest version of CGFX? After all your card and CGFX were originally made for each other.
Edit:
It could also be a hardware problem. Check so that you have all possible optimizations turned off for your card (IE gpu overclocking etc.)
Oh, also try turning off any patches such as FastBlit, MCP and so on that does stuff to speed up the custom chips.
