Keep Getting Fatal Errors

For the past month, my WordPress websites have been crashing. I keep getting a different “Fatal Error” Message. Currently, the message states:

Fatal error: Cannot redeclare z3n() (previously declared in /home/public_html/mywebsite/blog/index.php(1) : eval()’d code:1) in /home/public_html/mywebsite/blog/wp-config.php(1) : eval()’d code on line 1

It is now occurring several times a day and I cannot seem to get rid of the issue. Recently, I upgraded to 2.8.4. which seemed to correct the latest problem; however, now I am back to the same issue. It is becoming a great problem because the people that go to my site are questioning the integrity of my site.

Please advise on how to fix this issue. Thank you for your consideration and time. It is much appreciated.