Tom here are a couple of ideas to investigate. Back in the 50's and 60's when we had only OTA TV, sometimes if there was only minor RFI, like some lines, bars, and some sound effects, often the problem could be solved by simply making sure the OTA antenna was aimed in the correct direction, had good coax or twin lead, good connectors, and not too many splitters. If you have a good OTA antenna, try it on your TV and see what happens. Back in twin lead days, often the feed was very degraded, or often only one of 2 wires were still in place. A good strong signal into the TV may or may not override MINOR RFI. A pre amp at the TV antenna may or may not hurt or help your situation. If the problem is just overload (too much RF into the TV) this could be getting into the TV via the antenna, so a high pass filter at the TV antenna terminals might help. However since you are so close it is also quite possible that your RF is getting directly into the TV, not through the antenna connections.
Glenn had a good thought. How is your power maintained at 70 watts. If it is through the ALC circuits it is possible that your drive is set too high, there is an overshoot, and the the ALC circuit cuts back on the power after a short interval. I am not an expert in this area, it is just another idea to investigate. I would carefully read up on your FT-8 setup and make sure all is OK, however even if not it does not seem highly likely that this would cause VHF/UHF radiation from your TX. Is 70 watts an appropriate power for FT-8 with your transceiver ?.
Back in the dark ages, just demonstrating that a TX did not cause RFI to a TV set hooked to a portable antenna set up near the neighbors house knocked the wind out of the neighbors sails, and they either fixed the problem, lived with it, or complained to everyone. Try to be helpful, but think twice about touching the other persons stuff. Nearly 100 percent of consumer products are subject to part 15 FCC regulations, which briefly says that the device must not cause RFI and must accept RFI from any properly functioning radiator, such nearly 100 percent of ham equipment for many years now. I would print up a copy of the part 15 regulations, probably available from ARRL and/or FCC web sites.
I am not a lawyer, so my wording of the regulations may not be completely correct.
Good Luck Rick KL7CW