Error "Your system folder path does not appear to be set correctly"

[happycandle]

When I try to open random pages on my website I get this error

 

"Your system folder path does not appear to be set correctly. Please open the following file and correct this: index.php"

 

It's not a continous error because after I hit F5 once or a few times, it gets me to the desired page. What could be the problem in this case?

 

Need help asap.

Thanks