Bad Coax Cable?

Hello,

I have been fighting slow speeds to my modem for a few weeks now (paying for 600 mbps, getting 33 max when hardwired). I eliminated any splitters that I could, bring it down to just one splitter which splits between the modem and the TV box.   I called a few days ago and at first i had the automated help which forced a modem reboot, and the reboot fixed it for about 12 hours, but then i was back to 33. I called again the next day and the tech said I had an upstream/downstream problem and sent me a new modem. I swapped out the modems and the problem continues.  I tried putting the modem in place of the tv box temporarily and i actually get good speeds when it is in that spot, which makes me think it is the coax cable from the splitter to my modem.  I would just replace the cable, but it goes through several walls and up two floors on the back of the house, so its not an easy thing to do.

So two questions.  1) If this is a bad coax cable, does comcast replace those or do they only handle the ones up to the house?  2) If it is a bad cable, how was it able to get good speeds for 12 hours after the reboot?  FYI ive done plenty of manual reboots myself and did not have the same luck.

Any guidance would be appreciated! Thanks.