We have Comcast too...I am surprised you got an appointment so quickly the first time! When I call it always seems like there are no appointments available until a week from when I call.

When we were having problems similar to what you describe, it turned out ot be a problem with the line outside. Where the cable is on the pole (before it goes underground into our house) it was very corroded at the connector. THe tech replaced that, and then gave us a heavy-duty splitter for where it splits from internet to our TV (we had a cheap one there before). Our internet wasn't getting a strong enough signal, that's why it was so intermittant. So have them check the line, too. We switched out two modems before they thought of checking the line, what a waste of time! The modems were fine.

It's been over two years since we got those problems fixed, and Comcast has been great ever since.