Dropouts After Installing Wall Ports

I had a company come out and install three LAN ports in the walls. I provided the short cables that cross the final distance from the wall to the PCs/TV. Since then, I've noticed dropouts on the devices connected to 2/3 of the ports, but it's certainly possible it affects all three of them.

These dropouts vary in length from a few minutes to tens of minutes. During them, the devices indicate that they remain connected but have no internet. Often they specifically indicate a problem with DNS. Other devices on the network not connected through these cables are unaffected (these all tend to be WiFI). Running my own known-working cable to the TV fixed a persistent dropout tonight.

I want to learn:

• What is going wrong? Is it a problem with the cables in the walls, the interface at the wall ports, or my short cables? Or something else?

• If the fault lies somewhere in the installation, how can I prove it to demand repair?