I'm sorry, I wasn't being clear enough. I meant that when plugging my laptop in to the end of the 200' run it works fine on bot half and full duplex. Nothing still works if a switch is put on the end of that run.

I finally tested the one thing I hadn't before. I basically removed switch #1 from the equation, and put a little four port switch between the router and that 200' run. Oddly, the speed improved, but only to about 5Mbps down, and the upload dropped for the first time to something like 300Kbps.