GOG.com downloads failing-Airport with Comcast somehow the cause

I'm frustrated and baffled. I've had an Airport extreme for years (2009 model, I believe), and have had no known problems.

I've had Frontier for years, but this past Thursday activated with Comcast.

Downloads that work fine 100% of the time on Frontier frequently just "stop" on Comcast at a random point in the download...I don't get an error message from my browser (neither Firefox nor I.E.), they just sit there forever with the percent complete and the same time estimate.

Comcast is mostly working fine besides that, although I MAY have also seen this same behavior with Windows Update. It may be that this has nothing to do with GOG per se, but I'm seeing it lots with GOG because the downloads are unusually large, so there's a bigger chance they can fail. I'm NOT seeing it with streaming video, nor with Steam nor Apple downloads...HOWEVER maybe that could be because those programs have built-in error correction methods that would be able to tell something was wrong and reconnect?

I thought Comcast was solely to blame, but to my shock, the problem doesn't seem to occur when connecting a PC directly to the cable modem (A top-of-the-line Motorola/Arris that's a modem-only). It's hard to prove a negative, but I tried 4 large files and a few smaller ones, and usually I'd see this behavior doing that, but they all downloaded fine. (Now I'm annoyed that I didn't try a few more times just to be more certain...)

I've tried doing a factory reset of my Airport. I even replaced the Ethernet cable between my modem and router. Convinced myself it was working when I got a few more downloads done without issue, then it failed again.

MAYBE this has something to do with my Airport Extreme? Maybe it wast just dumb luck that none of the downloads failed when I was bypassing the Airport?

Anyway I'd LOVE any ideas/advice/whatever as I need to cancel one ISP or the other and I don't know what's going on. I've tried both OpenDNS (which works fine with Frontier) and Comcast's DNS, and the problem persists with either. There doesn't SEEM to be a widespread issue with GOG...this seems to be on my end, but I'm clueless :-/

I should note I'm doing this on PCs that are WIRED, though I never see issues with my wifi network on my airport either.