SOLVED - Port Negotiation on ISP Equipment
The cause of the problem was a port negotiation issue between the SG-1100 and the ISP Proxim Radio. When the Proxim radio was set to either auto or 1000 baseT the speed issue would appear, however when the ISP set the radio to only run at 100 baseT the problem went away.
This was a very odd problem, the issue could be consistently triggered by setting the radio to gigabit.
I feel like there is either a underlying problem with the ISP equipment or wiring to the radio.
Has anyone seen anything like this before?
Chris
Thank you @stephenw10 for your help.