I also think it is the flex connection cable.
I have a 2011 MB pro. I had intermittent connectivity losses after closing my MB when leaving the train and reopening it at the office. No networks could be found. A reboot would solve the problem.
Then after a while, I started to see 'No hardware installed' when I clicked on the (empty) wifi icon.
After replacing the wifi card, it worked for a while (so I suspected it was the wifi card). Then the problem came back. Now here is what I found.
The flex cable between logic board and wifi card has a connector on the logic board that sticks out a bit and which touches the case. I suspect that over the course of one or two years, depending on how much the MB is tossed around and used on laps, the connector suffers from pressure exerted through the case and the soldering points start to loose.
Once one soldering point gets loose I guess you will start to see unpredictable behavior - but all related to physical pressure on the back of your MB. One day it works all day (where you are actually sitting at your desk and not moving the MB around!), another day your wifi is gone. After carrying it around the connector will surely have disconnected a few times so wifi is gone, a reboot may bring it back once steady at a table top.
So the issue comes in disguise: we think it is sleep related - but it is actually related to what you do after you put it to sleep, which is of course put it in your bag and carry it around.
I came to this conclusion after carefully resoldering the tiny connection points of the flex cable (logic board side). (which is extremely hard, you need an smd soldering device plus a good magnifying glass and steady hands)
I also ordered a new cable which has not yet arrived. Mine had part number 821-1311-02 and I found one for $19 at PBParts.
Problem gone and has been gone since (one and a halve week ago).
I suspect that anyone who had the cable replaced, either as part of a card replacement or as part of a logic board replacement no longer suffers from this issue.
Also, it would explain why most suggested fixes seem to work for a while, with the problem jumping back in your face after a short while.
My suggestion to anyone who has these kind of issues is to order a new cable (not a used one!) and see if that works. It can be very easily replaced. Be careful when you take the old one off the logic board - mine was apparantly so bad that the second time I did that the connector came off the flex cable. So make sure you have your new cable at hand before getting the old one off.
Good luck to you all.