If you have a cat8 cable it is highly likely it is a fake cable. The cat8 standard is not even fully finalized and it is for cable used in data centers that have ports that can run at 40gbit. They are extremely expensive if they are real and they provide no benefit since your port can only run1 gbit anyway.
Ports dropping to 100mbps is almost always a bad cable. The only other option would be a bad port. Make sure you have cat5e cable or better only if they are cheaper. The key thing is the wire must be pure copper (no CCA) and have wire size 22-24 (no flat or thin cables).
The large problem lately are all the flat cables people buy. Most those have very thin 30awg wires. With the price of copper metal so high using...