First, to clear something up:
TW and Comcast got mixed up earlier in the thread. TW doesn't charge for diagnostic service calls; Comcast, OTOH, charges for everything. My bad on the mis-read.
The tech I spoke to online said that if it's determined to be my equipment that's causing the trouble, then it'll be $50. If it's their equipment, then it's free.
Back to your issue:
New modem (and, from Comcast), and 'net still goes down, modem admin page not accessable.
Can you move the modem from its current location, to the first termination after your cable entrance? This will (dis)prove your in-home wiring as being the cause of the issue.
The only Comcast cable jack in my home goes into the room where the modem is. There is a cable going directly from the wall jack to the modem. I can try replacing that cable (though it was provided by Comcast so I would think that if that's bad, then it's on them...but I could be wrong).
And, yes, it wouldn't be a bad idea to try connecting one of your PCs directly to the modem for testing. This will also (dis)prove the router as being the failure point.
Note that you'll need to power down the modem for at least 10 minutes before starting the 'test PC' (modem needs a power-cycle to reset itself.)
I'll be trying this tomorrow (or maybe Saturday). The problem is that the cable modem shares a room with my infant daughter (it's a small house so it's unavoidable) so I need to mess with it while she's awake.
At this point, though, I'm thinking janky cabling at some point (either in your house, or up to the pole.)
The signal values that you posted earlier are very acceptable. However, that doesn't mean that they aren't fluctuating. If you can't get at the mdem's admin page when service goes out, how would you know if the signal is still OK?
I think they are fluctuating but I'm not sure by how much. Too bad I don't have this modem unlocked like my old one (the one I just replaced). The only page I can access just has some basic info though it does include the power levels. I can't access the log or anything else. I still can't access the modem when the net goes down. I'll have to try unplugging the RG6 cable from the modem and see if I can still access the modem's admin page. That would also help removing the router as a problem, though I doubt that's the problem since it's only about 2 months old and this problem predates it. It's hard to say how bad it was with my old router since that one would also lose the wireless connection (which is why it was replaced) but I do know some of the connection issues had the same symptoms as they do now.
Anyway, I'll post updates as I try things.