It looks like your post has been split from the original thread that it was added to, so your comments have lost some meaning.
Apart from having to sign into iplayer all the time, what is the actual issue with netflix rarely working?
What happens with netflix? What onscreen errors are displayed with netflix?
Are any other onscreen error messages and codes displayed?
Why was the box replaced twice?
I tend to find that I need to sign into apps if I switch the 360 box off instead of leaving it in standby mode (using Quick Start or Active Start standby power consumption settings) or if the box has performed a system update.
Do you have any problems with losing recordings?
Does the box perform correctly if you pause a live TV program and then play it back?