A week or two ago, I got an OTA update for what I now assume to be the second 5.1.1 release, which I will henceforth refer to as "5.1.1bis", and references to just "5.1.1" will mean the originally released version. (By the way, thanks, Google, for reusing the same version number instead of giving it a unique identifier.) I assume that 5.1.1bis is intended to fix the StageFright vulnerability.
Since the 5.1.1bis update, my phone has been unable to stay connected to WiFi networks for more than a few minutes at a time. It will connect, then stay in exclamation mark mode for a seemingly long time (5s or more?). It then works fine for a while, ranging from 10s to a few minutes, then drops the WiFi connection, connects to cellular data, then reconnects to WiFi after a minute or two, ad infinitum.
This makes WiFi virtually unusable. The constant switching between WiFi and cellular data also, unsurprisingly, makes my battery life awful. It's so bad that one night my phone ran out of power while it was charging. I ended up putting the phone in airplane mode to charge while I slept.
This problem seems to exist regardless of the specific WiFi network. That is, it did it with my home WiFi, at friends houses, at restaurants, etc. It doesn't seem to have to do with any particular router or congestion or interference, since it seems to happen everywhere. In addition, my other WiFi devices at home don't seem to be affected. I conclude that it must be a problem with the phone.
The phone was in need of a factory reset anyway, so I did that. It didn't seem to help.
I then installed 5.1.1 via USB, which also cleared all of my data in the same manner as a factory reset. Since I did this, everything seems fine, but I remain vulnerable to StageFright.
I suspect that the radio firmware is flawed, but I can't seem to find any reports of anyone else experiencing the same problem. I suppose it's possible that the radio firmware update didn't apply properly, but it seems like, if that were the case, it wouldn't work at all.
I could try upgrading to 5.1.1bis again, but I've already reconfigured my phone twice and I don't really want to have to do it again.
Can I upgrade just the radio firmware to see if it causes the same problem under 5.1.1? If so, can I downgrade the radio firmware without resetting the phone?
If it turns out the problem is the new radio firmware, can I run 5.1.1bis with the old radio firmware?
Any other ideas?