This could be so MANY things. The first thing that I would want to know is what the signal amplitude coming into the modem is. Just satisfactory or a good, strong signal? An inline amp could help if especially if you have other devices connected to the same cable.

By many things, I mean it could be anything. For example, take a simple case. Suppose one of your cables (802.) running between your pc and router had an intermittent short/open or bad connector that a little vibration/movement caused a disconnect?? How do you find something like that? (this can be a bitch btw.) Try to isolate by moving cables around.

It could be a corroded cable connector or damaged cable (somebody drove a nail into it behind the drywall). If you suspected something like that there are ways to find these kinds of things. Ask Comcast if they have or/and use a TDR (time domain reflectomer). This will tell you precisely where in the cable (distance wise) the problem is. Another thing you might look at is the router's log(s). Sometimes that will give you a clue on what the problem is.

Like I said, what you described is not easy to pin down...