I know it's annoying, but I finnally found out what it is... the 500 error is when the SQL server is being hit too hard and there aren't enough resources to handle the traffic... it starts sending back incomplete or no data, and the browser or yourself has to resend the query. (hence the "two for one deal")
the server we are moving to will have enough resources to handle the site and the database for quite a while!
there is a limit to how much you can upload to a server (the computer where the website is stored on) at a certin time. Dave is waiting for this limit to be raised. since this is a big site, it has a big database, and it is easier, and safer, to upload the entire database at once, and not do it in halves, as he would have to do otherwise. doing it in halves is more difficult and also can corrupt databases.
i hope this helps (and if i'm wrong about something, someone please correct me..... the site I run is pure HTML and dosen't have any databases involved....)