I think it really matters sometimes on the TV and the DVI communications. Like one person suggested, I bet connecting it via Component cables would make a difference, but I really hate to use component cables (analog) over a single easy to use digital cable.
As for Brighthouse, absolutely, their biggest solution is to always swap the box, even when it didn't make sense. The way I got my 8640HDC was because my older SA8300HDC would only allow an external hard drive for 24 hours since it was new. I finally narrowed it down to the box so called to have it swapped out. They only had the Cisco boxes, so I went for it. Then I found out it doesn't accept an external drive. At least the 320gb HD is a step up from the 160gb.
Brighthouses second favorite thing is to send out a re-initialization signal. I had a weird thing happening with mine that no matter what I had the power on settings for, it always turned on and went to channel 0. I had power cycled and rebooted several times but to no avail. After 2 days I gave in and called them again. When they suggested they were going to do this I thought it would make no sense, but if it made them happy go for it. WELL, it corrected the problem. My point is, maybe this could resolve some of your problems.
Now that I've written the above paragraph, I have changed my mind. Here's my new opinion. The culprit/problem is probably more your TV and its handshaking.
As I posted in an early note, I am was not having the problems most of you are having with your resolutions. I had my 8640 connected to a Vizio 47" LCD TV via DVI. I had removed all of the resolutions from my box except for
1080i and I could turn everything off and on with no problems. A couple days ago the 47" went out, and while I wait for the warranty repair I brought my older 32" 1080i Vizio from the bedroom into the living room. The first thing I noticed is the TV and 8640 did some communicating, and now 480p, 720p and 1080i resolutions were selected again, and when I turn everything off and back on the 8640 goes to 480p for a second and then jumps to 720p. If I go into the setting and remove the 480p and 720p it remembers it and switches to 1080i and I have no problems until I turn off the TV and turn it back on. If I leave the TV on, I can turn the 8640 off and on all I want and it stays at 1080i, but if I turn the TV off and back on the 8640 switches to 720p.
DVI devices have to communicate with each other. Otherwise if you were to lock a device like the 8640 to 1080i and then have to connect it to something with only 720p, you'd have a problem. So I feel the problem has to do with the negotiations and the different variations of the DVI standard that are floating around. As in my case, my TV for some reason is telling the output device what it wants. It would/shouldn't be the other way around. Because my 32" is telling it that it likes 720p and apparently isn't telling it that it likes 1080i then that is what it is going for. I think it is these different variations that make this work correctly with some boxes like the 8300 but not with the 8640.
In case any of you mention anything about older 32" Vizio's not supporting 1080i, although this one is a few years old, it is not the model sold in Walmarts. It has PIP and does support 1080i and that is why I bought this version.
I also have an upconverting HDD DVR connected to the TV, and it always selects the 1080p output when I turn it on.
Hope this helps some of you.