I had a similar problem, Mac Mini would drop wifi intermittently and stop seeing the home network even though all my other devices were still connected and working. Restarting the router didn't fix it. Restarting the mac mini did fix it but obviously not an ideal solution. I discovered that if I went into network settings and chose Edit Locations from the Locations drop down, then created a "Test" location, all of a sudden the network was visible again. I could delete the Test location and it would still work on Automatic. This is still not an ideal solution if it turns out not to be a permanent fix for me, but may provide some pointers on cause.