NEED HELP!!!

Hi,
my comapny runs an application with mysql as backend.. Of recent every evening when you try to login it gives u the error 'unable to connect to server please check your connection'. If you shut down and restart hours later, it runs perfectly. if u ping d server when it generates the error, you get a reply and if you input a wrong username/password, it tells you invalid username or password.
Has anyone got an idea of what is happening..
thanks