In some surveys, I am geeting 500 - Internal Server Error.

What can I do to solve this?

As the error message says, this is most probably an internal server error, so check the error logs from within Cpanel, as the message states and see if you can find any more details about the problem. There is no way for us to know what could cause this with this little information. When does it happen? What are you / your users / your respondents doing exactly, when it is happening? What version of Limesurvey? But most importantly, check the error logs of your server or ask your server admin, what is going on...