Network card drops from 1 gigabyte to 100 megabyte every update

My Realtek PCi GBE Family Controller keeps dropping form 1 gigabyte to 100 megabyte every time I update Blue Iris. Any idea's?

bit ≠ byte
1 byte = 8 bits

How did you come to the conclusion that the speed of the link is changing? I doubt BlueIris has anything to do with it.
Crappy homemade cables are a strong possible cause. You can go into adapter settings in windows and force the link to be a particular speed, but if there's an issue with a cable this will cause problems.

Length of the cable and what is driving the cable in terms of the switch will also have a bearing on the overall connection speed that gets negotiated.

Some sort of bug in the diver for the network card or automatic windows driver update could also be to blame.