Yosemite WiFi dead chip
You may have hardware that is on it's way to being really dead.  The temperature of a marginal chip can cause it to be functional sometimes and non-functional other times.  When you resume from sleep, try letting the computer get warm, then try to enable WiFi.  The chip may need to get warm and then work.  This is obviously not a good long term solution but, if it works, it indicates that the chip (or the connection to the board) is flakey.