I got the same issue, even with the new update iOS 8.3.Please help! Just bought an iPhone 6, 16g. I updated it to iOS 8.3, but after the update, I had problems with the WiFi connection, which always drop off every 10 seconds. It never happened with my other Apple devices. After 48 hours, I went to the Apple Retail Store for a replacement, and fortunately, they replaced the iPhone 6 for another one right away. I decided to keep the old iOS 8.1 and not update it to iOS 8.3, because I was scared of having to deal with the WiFi problem again.
What's funny is that instead of having the WiFi problem, my new iPhone 6 constantly crashes after a blue screen appears. I searched the web, and saw that this kind of problem usually occurs with the old iOS (8.2 and lower) and the new iOS 8.3 is supposed to solve it. SOOO... I decided to finally update it to iOS 8.3, and at the same time I would test the WiFi connection. After another 48 hours of new iPhone replacement and with the iOS 8.3 update, my iPhone still crashes, and now it never comes back. The screen remains black forever even after many attempts of restoring it (home button + lock button, pressing them together from 10 seconds to 2 minutes). Even iTunes cannot restore it: ERROR 2009, 2005, 4005, 4014, 21, etc. Nothing works... but iTunes recognizes it as in "Recovery Mode". The apple logo or whatever logo never appears. I'm going back to the Apple Retail Store tomorrow, but I'm not happy because it's a waste of time and money.