Well, keeping trying, logging at verbose 7 level, no EE line, one WW line in log (mode clock 65MHz exceeds DDC maximum 60MHz) , whatever kernel I use.
I've tryied every NvAGP option, still the same: works on 2.4.18-14, not on 2.4.20.
Nvidia driver is of course reinstalled (with succes according to the log) every time I boot a different kernel.
Well, it seems that my problem is not AGP-related, but I still have no clue.
Keeping trying, keeping an eye on this forum...
Any advice appreciated.