From your given information, nothing in particular comes to mind as to the likely cause. One could offer some educated guesses, but you'd need more than that to take action.
It would help to know (a) the make and model of your antenna, (b) whether you have it on a rotor, (c) the type feed line you are using (RG-59, RG-6, or something else), and (d) what channels you receive consistently okay and which ones are causing your trouble (they need to be identified either by call letters, like WXYZ, or by channel number and city, like "channel 5 in San Francisco").
At first glance, 100 feet of feedline with a mid-point splitter might suggest to some that you'd do well with a pre-amp mounted on the antenna. But not if you're frequently reading 90 on your two marginal channels. Something else is going on.
If you'll update your post with the 4 points of info cited above, it might be possible to arrive at some sort of conclusion that could help you. While you're at it, include your Zip code in order to better analyze what you should and shouldn't be receiving.
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Followup per your added details. Based on your azimuth bearings and distance from the transmitters above (all located at Garner), I'm guessing your location somewhere around Durham. Close enough for this discussion.
Now, about your 4 channels:
5.1 actually transmits on the frequency of channel 48.1 with a transmitter power of one megawatt.
11.1 transmits on 11.1 with a power of 45 kilowatts.
17.1 transmits on 17.1 with a power of 12.47 kw
50.1 transmits on 49.1 with a power of one megawatt.
There is nothing in those facts that would suggest a cause for your problem, but there is a significant curiosity: you have two channels at the same transmitter site transmitting on two adjacent channels (48 & 49) with a megawatt each. Ordinarily, the FCC doesn't assign transmitters to adjacent channels in the same general market area, and especially not at the same transmitter site. This is most unusual, but I verified it with two resources.
That *could* explain some of your difficulty with your channel 5 (48)--adjacent channel interference from 50 (on 49)--your TV tuner isn't capable of adequately separating the two. That doesn't explain why it only happens sporadically, but it's still an oddity that can cause problems with most TVs.
As for your channel 17, the only thing I see there is that it transmits fairly low power, and it's at the low end of the UHF range. Without knowing what antenna you have, one can't evaluate its UHF performance, but maybe you have one that's better on the high end than the low end of the UHF band, especially with a channel that has rather low transmitter output.
It would seem that you have a VHF-UHF antenna, since you're getting good signal on 11 (VHF).
Bottom line: maybe something odd re 5.1 (48.1) due to its powerful neighbor on 49.1, and maybe a marginal signal on 11 due to their radiated power, your antenna, or both.
What to do? You didn't identify your coaxial feedline that I mentioned above. If it's RG-59, you'll get more signal into your TVs with RG-6. I'd do that first before trying an amplifier, if it's RG-59. If you already have RG-6, then I'd consider bringing down the antenna and replacing it altogether if it isn't one of the better models, like the 2nd link below (suggested for ch. 7 - 51 up to about 50 miles).
If you are using RG-6 and do have a quality antenna (properly installed, good conditions, etc.), then and only then would I consider an amplifier. The only way to do that right is to install a pre-amp, like the Winegard AP-8700 suggested by Rick. You do *not* want any type of distribution amplifier installed somewhere down the feedline or inside the house, as suggested in another answer. That will only give you problems that you don't already have. Good luck.