The channel 5 tower is by itself in location from you. It is starting to sound like an antenna reception problem. May be something blocking signal; trees, building, etc. Always had this problem? Do that check directly from the antenna. Aiming, moving antenna are some things to try. Also, it is usually better, if gain is needed, to use a preamp mounted at the antenna than an amp/splitter installed down the line. But with ch 5 supposedly your hottest signal (tv fool), overload of your amp/splitter could even be a problem.
And not all tuners are created equally. Some handle low/hot signal levels better than others and pretty much all newer tuners handle multipath/interference better than older ones.