waltereAny wisdom you can add will be welcome.
Would be happy to. 
The only reason I post here is to help people. I started on http://www.dslreports.com/forum/coxhsi and there was a link pointing here, and since then I have just been hanging out and helping. Its my hobby.
With that said, I'm not sure what the main problem is in your case. Like more harder to salve situations, its probably a mixture of several things. For one, I don't think the DOCSIS 2.0 modem helps. Its probably not directly causing the problem (since you have tried different modems) but even at it's best, I would be surprised to see above 23Mbps down and 3-4Mbps up. Also, if congestion is a issue, getting a DOCSIS 3.0 modem is a easy way around the problem. To see if congestion is a issue, try out this test. Here is a example of my results.
Detail for Test #152842
Summary
This report refers to a connection test carried out by IP address 68.228.152.xx on 03 Jan 2014 18:31:17 GMT-05:00.
| Download speed |  16258 Kbps |
| Upload speed | 12566 Kbps |
| D/load COS | 87 % |
| U/load COS |  - % |
| Min RTT | 15 ms |
| Max RTT | 19 ms |
| Avg RTT | 17 ms |
| RTT Consistency | 78 % |
| Max Delay |  38 ms |
| Avg Delay | 4 ms |
| Effective Speed | 71624 Kbps |
| Route Speed | 34952 Kbps |
| Forced Idle |  58 % |
| Route Conc | 4.4 |
| Download test | h |
| Upload test | h |
| Test | Bandwidth Speed |