[Sephiroth]

If you now have 20 dBmv downstream and you have 8 channels, under the 3 dBmv doubling rule (you'll have to Google that), the aggregate power hitting the modem is 29 dBmv which is close to the 33 dBmv DOCSIS 3 limit. Domestic cable modems are cheap items and in any case the DOCSIS 3 spec says that no single channel should exceed +17 dBmv. So tuner crosstalk, overdriven amplifiers and so on are failure conditions that you are experiencing.

My advice is that you either et a VM engineer to attenuate your modem input by 20 dB, or you go on eBay and buy two 10 dB forward path attenuators (85 - 860 MHz sensitivity); fit them in series on the end of your coax cable and all that will be sorted.