Finally, after a few months, I have a stable connection. Someone from Comcast called me to schedule an appointment to rewire my unit. They even sent a technician ahead of the scheduled date (without prior notice), but unfortunately I was about to leave that day.
So, at the scheduled day and hour Mr. Loren Knott knocked on my door. I should say I was disappointed by his first words, because I was expecting someone to promptly rewire the unit, but nobody had briefed him, apparently. Well, he is clearly an experienced troubleshooter and knows how to deal with frustrated customers. He said something like "I don't know what the other technicians said to you, but if you explain the problem to me, I can solve it" and promptly gave me his ID card and number to take note. I was talking to a pro.
He did everything that was possible at that time (replaced a wire section which was probably compromised, double checked the distribution box) and also gave me some informations about signal levels, instructing me to take a screenshot of the diagnostic page in case I saw some number out of range. I asked him if he wanted to wait a few days to see if those fixes could have solved the problem, and he said "we will monitor this, but we will rewire it anyway to be sure". Also, he said a full rewire is actually scheduled to be done in my apartment complex - yet nobody knows exactly when - but he was able to run an external temporary line, depending on my property manager approval.
What was really amazing is that he kept me informed during the next days about what was going on - and this is something I never had before from Comcast. Even when I texted him as he requested to inform about my connection status, he was already ahead of me and aware of the situation.
After a few days, the rewiring was finally done (and, even better, it was a permanent line, not a temporary one). It was about 1 and a half weeks ago, and since then my modem NEVER reseted again.
I have only one final suggestion to Comcast: it's completely unfair that, having so many unhelpful technicians, supervisors and customer service representatives, Mr. Loren Knott has not been promoted yet to supervisor, tech trainer, manager, or whatever title you can give him to help customers even more.
P.S.: Thanks also to @ComcastMelissa - I'm sure if it was not for her work monitoring social networks, I would still have a modem resetting every 10 minutes.
