PaulS is right. You should post your code if you want help.
I left the serial monitor open all night and in the morning I saw 250.000 lines in the serial monitor...
That is a lot. Is this going into a MySQL database? How many entries are there?
Here is a topic that covers PHP/MySQL and the ethernet shield.
http://arduino.cc/forum/index.php/topic,124289.0.html
Reply #13 has the php and reply #15 has the Arduino code I use.