You will always get this Error if access a lot of data via a WCF Service. The First thing people look at is the web config to change the max size of the buffer, but that is not a correct Place. To Fix open your service with a WCF client and at the bottom there is a webconfig. Change it to any number that is higher, i used 90000000 and there was no timeout anymore.
I hope this will help you. Normally you will get it if you are working with silverlight. because it is service orientated.