tunneling X traffic over ssh seems to fail at the four to six hour mark.
all the connections shutdown with little reported information. Normal
ssh sessions stay running fine.
autossh might help. It starts ssh sessions and periodically checks them to
make sure that they're still passing data-- if they're not, it terminates
them and starts a new one. This is often helpful, but maybe not in your
case, since you say that other tunnels are still good after the X tunnels
fail. I don't know what would cause that.