Issue with S 7 300

Hi

i have a s7 300 cpu used in a machine operation but we have an error SF led (red) is constantly on however nachine runs fine in this condition but sometimes the machine just stops dead and nothing at all works at such times the BF led also turns on. Then we turn the power of of the Plc and then tun the power back on and then BF Led goes away but SF led still remains on But as i said in that condition machine runs just fine 

Please suggest what to check for this issue 

thanks and regards