While UTP category cables are unreliable
Unreliable? Not hardly. I am confused because the second half of your comment ("I've rarely seen them fail") contradicts the first half. Ethernet cables are extremely reliable and can easily last virtually forever -
as long as the connectors are (1) crimped correctly to begin with and (2) not physically abused (yanked, tripped over, stepped on, etc.).
Just a thought so the OP doesn't have to spend money before realising that the cable wasn't the problem...
I agree 100% and thus suggested trying a different cable. Okay, you may have to buy or make one, but most long time users have several laying around.
Good point about auto MDIX (auto-sensing, self-configuring Ethernet ports that detect and configure for straight or cross-over cables) are
NOT, but not all network devices (hubs, switches, NICs, etc.) - especially older devices - support it.