Random 500 internal errors (more prominent when signing up for prosites)

Hi hoping somebody can help me shed some light on a serious issue which is preventing me from launching the service public.

I keep getting random 500 internal server errors, and again yesterday while testing the sign up process following making some changes to only the "text strings" on wp-signup.php.

Is there anything specific I need to be looking at? htaccess? reducing server loads? optimising wordpress?

Thank you in advance. L