Could even be the TV itself. Really can't say without more information, and you aren't being specific about which of the above suggestions you tried.
Since you never tested that leg of your coax before, you really don't have a baseline for whether it was passing great signal or mediocre, and whether or not the signal it's passing has gone down, or rather if the sensitivity of your TV's tuner has gone down.
Was the length of coax you tested with of identical length as the run giving you issues?
One thing I don't recall your having mentioned trying is, taking a TV that appears to be problem free and trying the bad leg of your coax chain with it. Preferably one with a signal meter built in for simple testing of various channels. For all we know, the signal chain has remained relatively the same and the TV tuner section has simply gone daft, becoming more sensitive, as jsmithepa has suggested.
Not having a proper signal meter, I generally use the built in channel strength meter on a ~19" digital TV to trace problems. It's crude, but with enough effort, you can generally track down what your average signal coming in on each station your receive is, and once you have a baseline at the antenna, you can trace your signal path and see where each station is being attenuated past viewable. If you're at the antenna and can't get the signal, unless it's outside and needs boosting, I suggest the antenna is the problem for that station. If you test your signal at each point in your chain, you should eventually pinpoint your problem. I've seen band-stop filter behavior from single legs of a splitter before, killing certain channels after the split, despite "working fine for a long time," so don't rule anything you haven't tested out. Even just the physical screwing and unscrewing of your F connectors could be causing your splitter to behave or misbehave.
Keep in mind, the tuners in every TV are going to have different sensitivities, so just because one set works with your signal level, doesn't mean another will be as happy. Heck, depending on manufacturing tolerances, and the low cost of equipment these days, I wouldn't even be surprised to see unwanted tuner sensitivity variations among the same make and model of sets.