The link on 100 Mbps seems to be unstable and frequently is established on 10 Mbps. The only thing I don’t understand why 951-2n shows me that it doesn’t advertise 100Mbps support in such case? Is it normal behavior when physical connection is not good enough?
This is going to be due to an issue with your cables between the devices. It could just be the rj45 connector or something else. If you can use a different cable and/or plug into another device (switch) - it can help narrow down the issue. There is a slim chance it is an issue with the physical port on the 951 though.