My PC randomly acts like the Ethernet cable has been unplugged, and sometimes reconnects at 100 Mbps instead of 1 Gbps. A different cable works, and changing router ports hasn't helped.
The existing Cat5e cable has custom connectors and runs about 10 metres under the floor, so replacing it won't be easy. Should I try reterminating it first?
Yes, I'd start with the connectors. The working alternate cable makes the installed cable or its terminations the main suspects. A bad contact or damaged pair can cause drops or leave the connection negotiating at 100 Mbps.
Test the run with a cable tester if you can, then replace the connectors and test again. A basic tester can reveal wiring faults, though a pass doesn't prove the cable will perform reliably at gigabit speed.
If the cable is solid-core and stays in the floor, consider terminating both ends at keystone jacks instead of putting plugs directly on it. Then use short patch cables from the jacks to the PC and router. That makes future connector problems easier to deal with.
If fresh terminations don't fix it, replacement is probably the next move. You might be able to attach a new cable to the old one and pull it through, but only if the existing run moves freely. Don't force it if it's snagged or fixed in place.
Interference from nearby power wiring or electrical equipment is another possibility, though the alternate cable test makes the installed run a better place to start. Check where it runs if you can, especially if the connector work doesn't help.
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