ParseError and SessionID Error

Hi 

I'm having a weird issue where some javascript/JSON queries will cause a 'ParseError' to appear in CRM.

However, the system log file shows: 'ERROR: Cookie Session Id:  does not match the Browser Session ID: '

The only fix at the moment is to reboot the CRM server to resolve BUT any ideas on why it then returns after a couple of days would be appreciated.

Thanks