Have a read of this:
http://www.skyuser.co.uk/forum/route...explained.html
With your terrible attenuation (due to the long line) I can't see there being much that you can do.
Do you at least use the master socket, rather than an extension? Have you ever tried the hidden test socket (behind the master's faceplate) to see if that makes any difference?
http://www.skyuser.co.uk/tutorials/h...er_socket.html
What do you mean by "flaky"? Is it unstable as well as horrendously slow?