Speed drops after 30+ days

Ok this is strange and it has been a problem for over a year with several netgear routers.

I have Xfinity 1gig now 1.2 gig internet. When I first get it up and running I get about 940mgbits

this holds out for about 30-45 days and the speed drops to about 400-500mbits nothing has been changed. After fighting Xfinity they send a tech out and he says everything is fine and his laptop gets full speed.? But I am still getting 400mbits?? so a day later bingo my speed goes back to 940+ hmm. so everything is fine for another 30-45 days and the same problem! so this time I hook up a laptop and it gets full speed? But my desktop does not. So I figure maybe the Ethernet on the desktop is going bad.I get a 2.5gig card and put it in the desktop and all is back full speed. About 30-45 days everything slows down again. I plug the standard computers ethernet in and all is back again full speed.

What is going on?

Now Xfinity is saying it is My netgear CM1150V modem or My RAX80 router and want to charge me to install thier Modem which has terrible WiFi.

I have a idea that it has something to do with some kind of QOS service but I have that off here.

Any Ideas?