IIS 5.0 HTTP 500 for css and images

I'm receiving a random http 500 error in the iis logs for
some images and css files, those files are served ok if I
try to access them from the browser and I'm not directly
able to determine the details of the error since it rarely
happens.
Anyone has any information regarding this issue?, it is
normal to receive this random errors (it is not a 500.13)

Thanks

Sebastian