When X crashes there will be messages dumpted into the log file which you can fine at /var/log/XFree86.0.log. Please check the file for errors (they are lines that start with EE) or for warnings (they start with WW). The errors nearer the bottom of the log are most relenant... but not necessarily the problem (look up around 20 - 30 lines in the file). Post any errors you find.
Also, grep the output of the command dmesg immediately after the X crash.. and look for mention of X, nvidia, or agp.
dmesg | grep -i nvidia
|