AH01990: PRNG still contains insufficient entropy

I have a problem with my apache server running on Linux Ubuntu. For some reason I have the following warning in the log file: AH01990: PRNG still contains insufficient entropy!. Sometimes I can access the website but most of the time I get the following warning. I tried to google this issue but I still couldn't solve this problem. It seems it is related to /dev/urandom. When I restart apache, the website is accessible for couple minutes but after that I get this error. I tried adding SSLRandomSeed startup file:/dev/urandom 512 and SSLRandomSeed connect file:/dev/urandom 512 to the apache2.conf but it doesn't work.