Hi, i've had the same problem, but oddly only for the last few weeks. I have a late 2011 Macbook Pro running Yosemite (10.10.3).
This is what i did earlier, and so far it's been fine, but i guess only time will tell.
Open System Preferences and go to Network (should be in the third row down) at the bottom of the Network window click on Assist me...
Then choose Assistant... Then just follow the instructions to set up a Location, the default name is the date and time, but you can change that.
Before the Location was set to automatic, which might have been the problem, but by setting up a specific Location for my/your Wifi/Internet connection means that it's not constantly checking for one, which is why it might keep failing. By telling our Mac's that our Wifi is the only one it needs to worry about, it should stay connected without falling off. But that's just a guess.
Hope this helps.