Slow down every 3-4 months - call Xfinity and they tweak some settings and its back(after trying to sell me rubbish)

This is getting ridiculous.....every 3 or so months my internet drops from 900 to 300. A reboot/power cycle never fixes. I call and first of the bat they try to sell me their modem. When I refuse, they say they tweaked some settings and it goes back to normal. This has happend 10 times now. This is criminal behavior.