This could just be a busy network issue.
Based on this ...
http://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/73566f83-c257-4941-8ed8-7ae45b2e7985.mspx?mfr=true ... the error can occur when a connection is established but no data is received within the timeout period. This can happen if there is a lot of network traffic and the client cannot connect to the server. It might connect initially to set up a connection and then not be able to transfer the data. The browser has a built in timer also ... if it cannot connect then it will wait a certain period of time and try again. After a certain number of tries it will time out and display an error stating that it cannot connect to the server.
If it is the same pages that are having problems then you could try and code the submission into a script on the page. Have it try and if it is not able to connect you can check for this and then loop and try again ... this would allow you to display a message to the user stating that the connection to the server is slow or that the server is currently very busy and that this might take a little longer than normal.