You’re aware of these tools?
https://www.dslreports.com/tools
When I was in my last house, I made the mistake of believing I’d be there for more than a few more years and wired the whole house for gigabit and put the 24 port switch in a structured wiring cabinet in the hall closet. I did this for like reasons: I wanted a movie server instead of shuffling memory drives.
There’s a chance you might be able to isolate your problem with those tools.
Now I’m confused: I just reread your comment and saw that you wrote the following:
“but the motherboard Ethernet still runs at 10mbs”
You wrote in the OP,
“Both network cards are 100mbs”
What’s the story???
One was on the MB and the other was a card. They both report to be 100mbs capable.