Only 100 Mbit/s over 1 Gbit/s network

The problem is likely in your cables. 1000BASE-T (gigabit ethernet on UTP) requires all four pairs of wires in the cable be connected correctly. If they are not, auto-negotiation will cause the interface to step down to 100BASE-TX (fast ethernet on UTP).

If even one of the two cables connecting the two 1000BASE-T PCs is not correct, then the two PCs will only be able to communicate with each other at 100 Mbps.