Slow internet speed

I have a problem with internet speed.  For years everything was working fine and about two years ago I noticed that the speed dropped.  I am supposed to have 500 Mbps speed but most of the time it is about 250 Mbps and sometime it goes down to 150 Mbps or even lower.  I had technicians three times over and what they were able to determined is that my signal is normal, on their instrument is shows 600 Mbps speed, at the very end of the cable, but when I try it on my computer it is 250 Mbps.  I tried to reboot modem and router several times with no effect.  However, when I restart the computer, turning it on does nothing, and I immediately check the speed it is usually around 550 Mbps but the consecutive checks are slower and after few minutes it stops at 250 Mbps and this is it.  I tried different Windows and Ethernet setups, nothing helps. 

My computer is new with i7 13 gen processor and Windows 11 Pro, the modem is my own Arris T25.  I checked the signal several times and all seems to be within limits with hardly any uncorrectibles.  I noticed that several times the was an attempt to update firmware but it was always unsuccessful.  When I talk to Comcast they tell me they can not update the firmware, but when I call Arris they tell me that only Comcast can update the firmware.

I am wondering if anybody is familiar with this type of problem and knows what can be done to improve it.