You could of course make the argument that you bought some combined cable modem/router/wifi AP box from them and it should "just work" - but you're just going to talk until you're blue in the face. Simply think of them as a pipe provider (not your IT support) and you'll be a lot happier.

What they provide is in general crap, and costing you money you can save off the monthly rental fee by providing your own equipment. This will also generally result in better performance.

I suggest buying a Surfboard 6183 (or lesser, depending on your area), and the wireless router of your choice. I recommend for simplicity sake the Apple Airport extreme or any quality 802.11n (or 802.11ac if your laptop supports it) router. Your network should look something like Cable from wall -> DOCSIS Modem -> Your wifi router. The only time you should be calling comcast is when the actual cable modem cannot obtain a lease and/or internet is in general failing.

Relying on Comcast to manage your CPE is just a plan for disaster. If your wired tests are getting 100Mbps, then the problem is on your end.