Sigh... this is still being a freaking PIA....
If it gets to the point where I have to type my pwd in before I can get back onto my machine, it seems to be fine.
But if it doesn't get to that point & I try to start using my machine again...
The AP will be sitting there in plain site, but the rMPB won't attempt to connect until I manually select it from the menu!
I haven't yet tried ALL the things listed, I definitely won't be doing number 10, even if all the others don't work!
I've since found that even a complete reformat & reinstall doesn't get rid of it completely... >.>
For a long period it definitely worked without issue, but now some sort of hw fault's creeped into the laptop, the AP, or there's more Wifi noise in the area than before.
I doubt it's the AP, as none of my other devices have had the same (or similar) issue, not that I've seen yet anyway.
First I'll rule out interference, I have some tools that'll help me devise a wifi config. that results in the least amt of interference for my AP/clients.
If there's still problems, I'll swap out my current modem/AP/router for a spare modem + AP/router I have.
If there's still problems, well, I think we've exhausted everything & can pretty confidently say my rMBP's Wifi client is sub-par, & must be placed on the service/support merry-go-round.
Still have this issue since I last posted, I've learned to live with it by having BT turned-off all the time.
Only when I need BT do I turn it on...
Sometimes I forget to turn BT off after I've finished using it & the problem comes back, so it's def. some kind of hw issue.
I could've gotten Apple Care to look at it ages ago, but I just can't afford to be machineless for more than 24hrs.
It's really not acceptable for such an expensive machine...
If anyone's observed this issue on other mid-2012 rMBP's & know how to fix or worked-around it, please share.
I hope to have a completely upgraded W/LAN infrastructure by the new yr, so maybe that'll improve the situation.
And I still haven't done everything I said I'd do in my prior post, time permitting I'll get round to that too.
So, it turns out that all along this really was just an issue with the OS!!!
I had an update come through about a wk or so back (10.9.4), & it's been completely fine since!
http://support.apple.com/kb/HT6281 (see the list of fixes)
Grrr so much time wasted on all this Oh well, at least it wasn't a hw issue or an issue with my WLAN.