I get the same thing. Had it with my iPhone 6, then went back to 8.4.
I then upgraded my iPad Air 2 to PB2 and receive the same issues. Can work for a few hours or minutes and then drop. If I turn off WPA2 and go unsecured it works fine, but then once I secure the network again it goes back to dropping out.
Only fix when it drops is to reset network settings and then it connects perfectly again.
I know its not a hardware issue because 8.4 works rock solid on both the iPhone and iPad.
Work around I am currently using is a guest network on my local wireless as I hope for a fix in the next release or so.
BTW I have a EA3500 Linksys dual band router using a 2.4ghz and 5ghz WPA2 Personal wireless network