EMI/RFI Injected On Cable Limiting Bit Rate

Not sure how to fix this.  Xfinity refuses to repair.  Old DOCSIS 3.0 line amp in pedestal, kluged to pass an OFDM channel over miles of 25 to 40 year old cable buried in swampy conditions.  Line amp diplexer has drifted so the guard band knee starts at 38 MHz, buried in noise by the 40.4 MHz center frequency of channel 5 DOCSIS 3.0 upstream (low-split).  Both my modems scream a 55 dBmV channel 5, though I got that down to 51.8 with a short, dedicated run.  SDR shows other 3 customers on my tap have similar upstream problems, particularly and peculiarly at 41.5 MHz.

The SDR shows an unconnected open-air noise floor of -80 dBFS.  Connecting the feed to the SDR immediately raises the noise floor to -60 dBFS.  The signal is ratty with injected spurious signals, but the 32 DOCSIS 3.0 QAM256 channels do quite well despite a 4.5 dB up slope as freqs increase.  However, the kluged OFDM channel looks like a well-used mine field.  Steady state has 5 to 8 dB spikes/valleys on a kHz to kHz basis.  There are cell-related spikes everywhere, often creating 10-12 dB divots in the waveform about 150 kHz in width.  Dozens.  At 743 MHz, signal begins to weaken until 15 dB down at 744 MHz.  It then rises 15 dB up at 745 MHz. where is slopes off another 4 dB over the next 250 kHz.  There's another 10 dB divot at 750 MHz, 1 MHz in width.  At 755.5 MHz, signal slopes off 20 dB by 757 dB.  This should not be edge of band for a 722 +/- 48 MHz carrier.  And indeed, it slopes up to a 15 dB "AM"-style hump at 761.850 MHz (cent. freq.).  Then gradually rising 5 db until 763 MHz where it jumps 10 dB and sustains with usual 8 to 12 dB cell phone dropouts until presumed carrier limit at 785 MHz.  However, carrier drop off should have been at 770 MHz.  Also noted the described 20 dB lulls are only 10 dB above the Xfinity noise floor.

On the OFDM channel, I average 1.5 million corrected errors per minute, with maybe 15,000 uncorrectables received over several days.  The OFDM does lock, but it uses all 4 profiles (0, 1, 2, 3) meaning the modem is using both slow (DOCSIS 3.0) and fast (DOCSIS 3.1) profiles, most likely varying over time.  

The result is a highly variable bit rate on my new 1.0 Gbps plan (was rock-solid on DOCSIS 3.0, 400 Mbps plan).  New actual bit rate varies from 200 to 950 Mbps with noticeable response latencies, dependent upon test and PC used.

Xfinity tech tiers 1 and 2, (AI and human techs) both swear there's nothing wrong since the average power on the OFDM channel is 6.58 dBmV and nearly all errors are corrected.  First, the Motorola MB8600 modem can only pass 850 Mbps, not the 950 it's certified for, which is the reason why I only get 500 Mbps (?).  Then the MB8600 is broken, right after being told everything was fine with the modem specs.  Finally, the brand-new Netgear CM3000 must be broken right out of the box because it did the same thing.  

To a Layer 2 guy, everything may look OK.  To this Layer 1 through 4 guy, it's a different story.  25,000 errors to correct every second takes a lot of processing time.  Not only does the modem processing slow things down, but my network card and CPU has a whole lot more to assemble.  Then when it does finally assemble a coherent message, it sends out an ACK.  If the ACK gets filtered out because 40 to 42 MHz is filtered or it's just too noisy with injected spurious signals at 7-12 MHz, the sender/originator of the message not only has to resend it, it will see the delays as congestion and slow its transmission rate.

3 van techs came out to show me where my newly upgraded cable/network system was failing their signal, but the best they could do is replace a connector in the pedestal (see photo).  Finally, a line tech came out who verified what I previously found, and said there's no way they're going to replace the failing line amp.  But maybe this summer they will upgrade to a true DOCSIS 3.1 line amp.  As one might suspect, I'm not very happy I've lost one modem to overpower conditions, have a replacement show signs of thermal stress in just 6 months, and purchase a new $300 modem unnecessarily.  I'm also not happy I can't put a splitter on my network for cable TV, and it's not right I have to spend hours adjusting network card and registry entries on my PCs/laptops to reduce processing loads and times.

Does anyone have suggestions how to get DOCSIS 3.1 advertised speeds on this kluged/RFI-infested system?