I am in very similar situation for past few weeks. Coincidently it seems other in this thread from South Florida as well. My speed tests generally produce good results and when I download large file like Steam game I get solid results.
Perhaps anything that require real time connection is terrible right now, worst part it comes and goes at random. Pingtest to Comcast Denver server varies dramatically, could get few A’s in a row that will follow up with five D’s and F’s . Usually when I get bad grade its pocket loss from 3% and up. All this makes playing any game online nearly impossible.
Have tried changing to a different modem and connecting pc directly to the modem with pretty much same result. Numerous calls to tech support also did not produce any results as we do the same old song and dance with power down, refresh signal etc. Any suggestions would be greatly appreciated.