In my experience, a 100% working solution would be to shell out for an airport express, or an Airport Extreme Base Station (a used, previous gen, base station will suffice). If you don't wish to continue the hassle of plugging in the wires, resetting iTunes and the ATV all the time, that is what I recommend.
Simply create a wireless network on the Base Station or Airport Express (it doesn't have to be connected to the internet, mine isn't) and then connect both the ATV and your Macbook to it. It works flawlessly, and you don't have to tinker with the network settings.
Reason for preferring Base Station - if you attach an external hard drive to the Base Station, just move all of your media files onto it, or any other files you feel take up too much space on your computer, and tell iTunes to read from that Library. I'll post an update on the method but as my setup has the Base Station disconnected from the internet, I'm attempting to connect my computer to both the base station and my other Belkin wireless router.
The only thing you may be giving up is Netflix, but as their service has been on a steady decline, I don't see it as much of a loss; but if you need it, simply go back to the settings in the ATV and change the network to the Router connected to the internet, and you can browse Netflix. And with the new version of OSX set to release in the coming months, you will be able to Mirror your Macbook to the TV, wirelessly, without 3rd party software.
Also, I had a similar issue, I could connect to the internet, but my iTunes Sharing was garbage, dropping constantly, I was so frustrated until I used the methods outlined above. It's your router, I tried many posted solutions on how to edit your router settings, but none of them were fully successful and required more work than needed. So then I tried this on a hunch, and bam, instant HomeSharing Availability, it's been a month, never a single problem like before. The only thing that might stop you is the price between 79.99-99.99 for the Express or 129.99-179.99 for the Extreme Base Station (for Hard Drive Sharing Purposes), depending on whether you want a used or new product.
Just my two cents, Best of Luck brother.