Interesting. Sounds like you're on the right track.
FWIW, I have all the ports on my Actiontec router left to their default setting, Auto. Connections that warrant 1000 Full Duplex get it, and others that are only 100 Full Duplex get that too. I always get my 75/35 plan, plus a little, when running speed tests from various devices. You might try using those default settings for a day or so to see if they help any.
Or...if locking your PC to 100 Full solves the problem, you can't argue with success. :smileywink:
Please do report back if and when you solve the problem. I'll be curious myself and it may also help others with a similar problem in the future.
Good luck.