I have tried older drivers, and finally I reinstalled the driver 1.0-4363 (with the installer). I put the Option "NvAGP" "0", and it seems to work !!! I'm currently writing on the forum with Mozilla. It didn't crash on start, and it seems to be stable ! (before, it wouldn't even load the home page).
Well it seems that something goes wrong with Linux 2.4.21-0.13mdksmp i686 + Riva TNT2 Model 64 + driver 1.0-4363 ! (and also some older versions).
Woohoo, that works!!!
Of course I'm lucky, I think of persons like WaxyLemon who already tried to disable AGP, without success
Now, I'm going to try "NvAGP" "1" or "2", in case one of them works.