Start with removing any amplifiers, signal attenuators, or splitters from the coax.
From there check the line for kinks, damage, moisture in the line.
Check the connectors for improperly made ends, foil touching the copper coax line, loose connections, bad/old/cheap connectors, or corroded connections. Replace them if you do.
If you can, simply connect the modem right where the coax comes into the home. This prevents wiring in the home from being the issue. And some ISP’s charge if the wiring issue is in the home. So this helps prevent this.

Xfinity cancelled my appointment today because they claimed since everything is working on their end, then I must not need any assistance. They decided they didn't need to actually tell me this, they just didn't show up. Second time they have done that to me.

So that got me thinking about what else I could do. I actually removed an amplifier from the situation. Xfinity installed that a while back. They actually just got done installing new amplifiers on our street this past spring. 

I went straight to where the line comes in the basement, uncoupled the current line going to the amplifier and coupled a new coax to it that now goes directly to the modem. I don’t think I have had any issues all afternoon, so time will tell. 

Xfinity is now supposed to come Monday (for real this time). I am still going to make them do a good check of all my connections and signal strength.

Thanks

Hello billin30

                       After checking the signal levels with the ISP you can get an idea of what causing this issue so just monitor your connection and check on the cable signal level 

                       http://arris.force.com/consumers/articles/General_FAQs/S33-Cable-Signal-Levels