And the culprit is ...
I was so happy yesterday to found a solution to the General Network Error.
However this morning, I still received few of them, so I decided to find the culprit.
And I got it, it's the Data Application Block I use in many cases. It's a great stuff to have an SQL helper, but it seems for me it's a bit buggy.
After replacing my code with a classic datareader, everything is ok now. Maybe the helper is too chatty :-)
Now I have reduced by half the number of 'random' errors. It seems that if I force a dispose method after closing the connections, I have a better result.
Unfortunately for some unknown reason, I can't use a dispose in the method I use to generate an image from a database. If I do, I have no images at all. I can close the connection, but nothing more.
I know it's not perfect, if the error happens, a broken link appears for an image, but at least the page content is shown, and that matters.