I'm expereicning the exact same issue, only, I just recently bumped my internet from 200 mbps to 600 mbps and haven't called tech yet. Oh, and I'm using their gateway, it's the newer white colored one. I increased to 600 mbps because my wife is teleworking and my kids are both in virtual class all morning. Since I increased it to 600 mbps all three have horrible service, audio cutting out constantly, it's unusable and I've had to pay for a hotspot for my wife from Verizon wireless, which is ridiculously expensive. I dread calling a tech at xfinity because it's such a huge time waste and I'm very busy, my day begins at 4 AM and I don't get a chance to stop until 9:30 each night, so spending 2 hours to talk to a tech and have it do nothing in the end is somethign I don't have the energy to subject myself with. I wish xfinity would at least respond here, but nope.
Im curious about something at hardwired level.
Login to your modem
if its an xfinity modem should be that 10.0.0.1
Goto Gateway -> hardware -> lan
Each port has a Connection Speed
It should hopefully be 1000, if not possible causes could be poor quality ethernet chord, damage ethernet. Device connected to the port may have bad settings, or has a hardware limitation that doesnt support high end speeds.
I am having the exact sam problems. 1 tech did exatly what yours did. changed out the box abd ground wire on the ouside of house. conectons to wires out side and at the pole. the next one shanged the wire from the box out side to the in side plate said therwe was loose conection inside the wall. and changed a filter out at the pole. it got worse. now have a third tech comming tomarrow. on the phone they had me call aris and there tech told me there is substandred conectin comming to the house. witch means there equpment out on the poles are flaking out. i have lived at my address for over 20 years now and was first one to get intrnet on my street. that same box and lines have been there for at least that long. most likly they are going to have to upgrade some of the older equpment with there new higher speeds cause it cant handle the loads it is trying to keep stable. not sure. but will inform what happens tomarrow when the 3rd tech come to battle this problem. I have the new aris sbg8300 its capiable of 10 gig. but getting numbers just like you are getting. I live in chickamauga Georgia. also. wireless is a joke also. reseting modem at least 1 time a day some times 2 times.
thats what the aris tech had me do and call out the down and up load numbers. thats when he told me there siginal was problem wasnt any where near where it should be.
Oh I upgraded form the blas 450mbps and it was working fine then would see numbers as hi as 600 my self. and I also shut off the cable subscription. seem that if you are going to just use internet. you go through this problem? jsut a thought.