In the past month or so, my M has started losing connection frequently on the 2.4Ghz band. I have done multiple tests on my home router to see if it could be the culprit, but the issue repeats itself on other access points and routers in other locations as well. I will go from a full connection to the WiFi icon being completely gone and my phone cycling back to 4G.
When I am able to connect on the 5Ghz band, it appears to have to "warm up". Download times will list in the "hours remaining" set before finally picking up speed.
I have isolated this to a hardware issue. The issue is replicable on the April/May 4.1.2 update, the July update, and multiple variants of CM and AOKP ROMs.
Is my phone on its way to the great Cell Tower in the Sky?
However, you have to walk a fine line here. If you get a a warranty replacement, there's a good chance you will get an updated phone that will be impossible to unlock the bootloader and difficult to root. You might just want to see if you can get it repaired.
