Constant service drops since 4/26/12

Our Internet service has been effectively unusable since 4/26 which has made it practically impossible to conduct business. We've had multiple service calls to Comcast support, tech visits, 3 different modems, conversations with supervisors, several firmware upgrades including test firmware and still no resolution. This is what I know.

on 4/25 and 4/26 there was some upgrade to Comcast's system involving multiple changes. After that our service began to fail. I've run continuous low impact ping tests against the first hop router and discovered that very consistently, connectivity is up for 2-3 minutes, then down for 2-3 minutes. I've been monitoring packets on the network and saw a continuous flow of ARP traffic from Comcast's network into mine. These were for IP addresses that had nothing to do with my subnets, but proved it was not a wire-level connectivity error. There was always a flow of packets. The connectivity problem appears to be at layer-3. At one point we had two modems in parallel of a splitter one using my static IP, the other a DHCP address. When one failed, within a very short time the other failed. One tech said that there was a problem syncing on 2 upstream channels. That would make some sense why layer-2 could flow to me - no reply involved.

A few times during the month service came back for typically about 8 hours. This is completely anecdotal, but what is weird is that it starts working at 10am and stops just before 6pm. A couple of times it looks like it was up all weekend. I'm wondering if our service failure is due to some other party with a defective modem. Also, a few times established Skype calls were able to continue for quite some time after we lost web access and shell connections.

We have had Comcast business class Internet for about a year now. This was a new install, new cable installed from the street into our building. We only use the Internet service and until 4/26 we have had rock solid, reliable connectivity. We've had to use Verizon 4G tethered off our iPads to keep working, which is expensive, but fortunately has been working as long as we don't run out of data-plan bytes mid-session.

One frustration is that I get no feed back from support telling me what is going on. When the service seems to come on, I don't know if it is due to random factors, or some change on Comcast's part. It started working today (Sunday) so I have no faith it will be on tomorrow morning. Did they fix something? Do they think it is fixed after every change, and wait for me to make yet another call to tier one support to complain? I've had more than enough of those to break my contract, but until recently I've been please with the service. I've begun to look for alternatives. Megapath and Telepacific seem to offer cable broadband in my area, but as it used Comcast's wires and ??? before it gets into the alternative carriers' networks, I'm not sure they can provide better service.