Some ideas to try
Bman72s
I have just enough (outdated) expertise to be dangerous, plus I am paranoid about signal overload, plus I obviously don’t proficiently speak Canadian… so it took me awhile to figure out what you meant by “CFTO”, but I'll try to help.
As I'm sure you know, your 4221 antenna is a 4-Bay Bow Tie designed for UHF reception. CFTO is broadcast on VHF channel 9 @ only 17.4 kW of power. It also comes from ~43 deg. off bore sight from the Baltimore (Fox) stations. One source of your trouble with CFTO could be that a combination of propagation anomalies and antenna pointing are responsible for your degrading reception over time although I would expect CFTO to appear randomly good or bad. A solution to that would be the addition of a High VHF antenna such as an Antennacraft Y5-7-13 or Y10-7-13 and aiming it towards the Toronto station, then optimizing your 4221 for Baltimore. If your “original” amp has a single RF input, you would also need a UHF-VHF Band Separator/Combiner (UVSJ).
What you describe could also be a milder case of what I experienced with my OTA project. Because of the intermittent nature, I first suspected bad cable connections or parts. I replaced an existing ~30 year old; 125’ run of RG-6 with brand new RG-6, fitted with new Channel Master compression connectors. I substituted both the antenna to pre-amp balum and its jumpers and tried different pre-amps. Sometimes it would work pretty good, at other times it was flaky as all get out.
The 7778 pre-amps are excellent in weak signal environments but lack the “head-room” to tolerate moderate to strong signal environments. Antenna pre-amps are designed to amplify signals within their “linear” range. When input signal strengths exceed that range Inter Modulation Distortion (IMD) products are produced. IMD products are mathematical sums, differences and multiples of the signals involved. If your 7778 is being hit by strong signals causing gain compression or whose IMD products fall on CFTO then that could explain your erratic reception.
To test for this, remove the amplifier and power inserter from your lead-in and see if the CFTO signal can be received reliably (if at all). If it is received but variations still occur then some other mechanism is at work, probably multi-path. 50 years ago we used to get this near Lindberg field as the signals reflected off the arriving & departing airplanes.
If you can’t receive CFTO without a pre-amplifier, try holl_ands trick of using 3 – 6 dB attenuation between the antenna and the pre-amp input. Again, if the degradation is eliminated it is indicative of 7778 overload, if not, it’s some other mechanism.
Obviously, while your making these changes, re-check for loose connections or intermittent cables but they would typically affect all channels.
Partially removing the coax from your TV should reduce the signal strength. One last thought is that your 7778 has to much gain and is slightly over driving your TV tuner. Since most modern tuners are designed to work with relatively strong cable channels, and you've tried two different TV's I doubt that that is the case, but it wouldn't hurt to try your attenuators @ the TV input if my other suggestions don't work.
Please let us know what you find.