dapper freezes or reboots without warning

i could use a little assistance please. my install of dapper has recently taken to either restarting the graphical interface or freezing completely, requiring a hard shutdown.

i don't even know where to look to see what is the problem; is there an equivalent of windows event viewer? how can i tell if it's x or gdm? would reinstalling the gnome-desktop meta package refresh all the files, if one's corrupt?

looking forward to hearing from anyone with a clue!