"500 Internal Server Error" may result from an error in your CGI script but it may also result from a fault in your web server (apache?, IIS?, something else?). If the server is returning a 500 status to the browswer but not logging anything in its error log then I would say that it is broken or misconfigured. You should fix the server so that it logs useful information before struggling with possible CGI script errors.

The fact that a script containing nothing but a print of a hard coded string sometimes fails suggests very strongly that your web server is broken.