I've been having constant dropping wifi and inaccurate gps signal for a few months now on my iPhone 6. I had it reset 3 times, nothing seemed to improve it. It was an intermittent problem sometimes it worked fine for a few hours other times it didn't work all day. I took it into Apple yesterday and they were amazing. My phone is not under warranty with them, I purchased it from eBay second hand, however I was told because of consumer law (this may only apply in the uk) for the first 2 years the company who sold the phone (in my case it was originally purchased from an Apple Store) have to replace any products with faulty softwear issues free of charge and was given a new phone there and then.
I Know this isn't a conventional solution but if your phone is less than two years old I recommend you looking into this with whoever you purchased your phone from. I'd never heard of this law myself and it was only the Apple employee who told me about it meaning I finally now after 3 months have a working phone again!