Hello, I think I may have found the problem at least for Verizon customers.
Let me begin by saying I got my new S5 yesterday, loaded it all up and started noticing during that time that it would drop the wifi. Didn't think much of it until my wife said it was doing it the next morning and said we need to get that fixed. Hour and a half later, I've exhausted all of tech supports knowledge, and I get out of the guy that he sees there are other reports of this. I said "Yeah, do a search and these forums know all about it and are saying unless you carriers inform Samsung, they won't know there is a problem. " So we get Samsung on the line, and I am still flabbergasted that an employee of the company, being given this knowledge, says they can't do anything other than take the phone in for 9-11 days to repair it. I said " How are you going to repair it if you won't acknowledge you know there is a problem? There is evidence on this three way call from the carrier saying there are other reports... " ...long story short, I'm not waiting for a fix from Samsung any time soon.
Here is what I found when I went to exchange my phone at the Verizon store. The guy set it up, we had connection, and it held. I went home and loaded all my apps, and WHEN I WENT TO ACCESS THE MY VERIZON ICON, to put it on my home screen, it immediately kicked over and said "switcing to mobile data" and started the process of dropping and reconnecting to my wifi every couple of minutes.
I did a factory restart, loaded everything on my phone that I wanted, but did not touch, nor will I ever touch, the My Verizon icon, and I have not lost a connection yet. I have informed Verizon of this development, and they say they have noted it in the account. Sigh...
Anyway, I hope this helps, and leads to a fix one day...!