To start with you must be able to demonstrate that it is "the wires".
You do that by swapping in long, known working (at speed, no spikes or packet loss) cables.
Usually that sort of testing involves running long Ethernet cables about the house from Point A to Point B etc...
You need to be methodical and allow enough time for each test run to get a good "sampling". Especially with intermittent problems.
If the problems stop then it is indeed likely that the issue is along the actual wire path from Point A to Point B that you bypassed.
Then, within that path you can take a closer look at cable run (hopefully), cable terminations, and wall jacks.
Stringing test cables about is not always a viable option if there are kids, pets, senior citizens, and just about anyone around who could get tripped and/or cause damage to the test cable.
As for a specific fix that would be a TBD. A poorly terminated wall jack is one thing. A damaged Ethernet cable behind the drywall is another. Or maybe even substandard cable was run in the walls.
Make note of what is printed along the length of any network cables you can find and see.
Does the house have a patch panel and wall jacks? Some photographs may prove helpful.
Post photographs here via imgur (
www.imgur.com).