Coming in late here but in my arrogance I think I can add a bit. HFAC connections are tricky. The key quality at the TV is signal/noise since most devices (I believe) have some amount of amplification and AGC.
Since you say the situation
became worse i.e. something changed that suggest one of two possibilities. Reduced signal or increased noise. Here's my checklist on possible causes (anyone chime in if I missed something).
Possible sources of noise are:
- Noise in the original signal from the provider.
- Noise occurring at connections due to non-linear behavior (oxide coatings on connectors can cause them to behave differently.. the most extreme case being a spark gap which conducts only after voltage rises above a threshold.)
- Noise from external EMF leaking in at connectors.
- Noise from other devices connected to the circuit.
- Noise from external EMF leaking directly into the cable. (They're shielded but kinks or wear can defeat shielding.)
Possible sources of signal level loss are:
- Reduced signal from the provider.
- Degredation of the cables (unlikely, even with cheap cables) [Edit: that is unless... I've seen old cables where water seeped in through cuts in insulation or at connectors and oxidized the shielding. They looked fine until the insulation was stripped off. Look for cuts or connections exposed to moisture/weather.]
- Signal attenuation at connectors.
- Signal leaking out at connectors.
- Line reflections at connectors.
- Line reflection along cables (at kinks or damaged areas).
- Diffractive interference due to change of cable geometry. (Can be very frequency specific, signal at one frequency creates a standing wave and your TV happens to be at a node.)
- Signal loss at other devices connected to the circuit.
I think I've covered most bases. If I were the OP I would first try to remember exactly when the trouble started and query (very politely and non-confrontationally) other residents to see if they have added or upgraded any devices connected to the cable system. This could have added noise, reduced signal, or (less likely but conceivable) caused an odd standing wave issue. The former could be solved by having a filter or attenuator placed on the new device which doesn't affect its performance but removes the problem. I would first try to eliminate other issues.
The less likely but possible geometric issues may possibly be resolved by changing the length of cable to the problematic TV. Try adding a 6ft extension and see if the reception improves. (very unlikely but would be definitive.) Alternatively if you have enough extra cable, cutting of a bit and adding a new connector.
My first course of action would be to inspect the entire run of cable or as much as possible. There may be kinks that over became pronunced enough as the plastic dielectric deformed to cause the problem, or there may be wear spots or cuts at some point where the cable was mounted.
I would next check each connection for oxidation. Try lightly sanding and tinning copper axial connector with a bit of silver solder. Be sure to clean off all flux afterwards.
You could then try to eliminate any incomming noise from connectors. Simply wrap them in aluminum (or aluminium for you non 'mericans) foil one at a time and checking to see if the problem goes away.
Note that if the problem is comming from beyond your apartment then a booster may just boost the noise as well as signal. If the problem is also not from your provider a booster at the main connection may solve it.
Some noise can be harmonics of a different frequency signal leaking into the system and could be eliminated with a band pass filter. Something like that could be comming from a device in your appartment so also try to correlate any new equipment with the timing of the problem's onset.
Other than these you could speak with the provider and see if they could boost or have recently decreased your signal. (I believe they try to keep the signal strengths "just enough" plus a margin to save on power and equipment degredation.) You could also ask them if you could speak directly with one of their technicians who could give you some more direct advice based on their experience.
Hmm... what else? It could be gremlins or other supernatural entities. Get a good mystic for that one. I can't help you.
Final note: My expertise is theoretical physics and not cable tech. I'm thinking in general terms and have no real experience to weigh the probabilities of these various cases. I hope my late post has not been too redundant from earlier advice and proves helpful in some way.