A few years ago I had this problem. Well, actually my Aunt and Uncle did.
The problem usually stems from the wireless being the primary
use of the router so therefore it will put more effort into providing a stronger signal.
I simply went out and bought a splitter. Split the signal 5 ways. One cord going to the dedicated Ethernet computer, and one of them going to the wireless router. Seemingly, no problems have been present since.
Best of luck,
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