I had the same problem.
Comcast kept telling me it's on my end.
They must have a 5 ft high stack of complaints from me by now.
however, a sharp Comcast inside tech identified the problem as being in the outside wires.
He made an appointment for a comcast outside tech and lo and behold, they found 2 things wrong.
1. a squirrel had partially chewed through the wire.
2. a connection\coupling was encased. The encasement had filled with water. ( Makes Sense that the service was spotty, when you consider the continuous freeze & thaw)
They replaced the chewed wiring, sealed the encasement and my speed is now screaming.
You have to keep hammering ‘em to get a decent outside guy out there. preferably a comcast guy not a sub contractor
Good luck
Yeah - I’m in the 95th and Cicero area and I have Comcast - I’ll have to try them again and see what happens.