Same issue here as well (HTC Desire 601)
I've wondered if "wifi hardware is affected by environmental conditions." - phone internal temperature
In my case WiFi appears to have issue starting after a phone reboot when the phone is very warm.
lets face it - some of these apps suck and suck up the power - runaway app, bad design, or whatever...
I find myself rebooting the phone if (after shutting all aps) the phone remains hotter and battery usage higher than normal.
but very often (not always) the wifi will not start after Android restart or full shutdown and start.
Eventually, after multiple attempts the WiFi finally starts up and works normally again.
I've found better luck if I let the phone cool to room temp, then start it up.
Which makes me agree, that the WiFi component temperature tolerance appears not to be broad enough to function normally at some of the extreme operting temperatures of this model.
Or perhaps its the other way round - the WiFi components itself running away causing the heat up sucking up battery (in my case a Pocket Warmer) and the WiFi not starting is a side effect of this condition... ??
Just a theory.. ??
Is anyone else's issue associated with a WARM Phone?
Try to let your phone fully cool down (powered off) for an hour.. restart and try again...
if it works normally after cool down (and no other changes) please update this thread.