The yellow light shows an active connection, right? If the green LED is blinking it does. If not then there are four wires, one of those could be disconnected and the amber light will still be lit.
Trust me, a cable tester that shows 1 to 1 mapping can resolve this in a second....And yes I've seen bad wires from Netgear and LinkSys right out of thier freshly opened packages.
Try the wire to verify it works.......Always start with the simple stuff first
Is the NIC a Gigabit one? And the switch a 10M, check the duplex speed....