MBP 2010 can't connect to wifi 5 Ghz

Does it reconnect if you disable WIFI and reenable it after waking up?

Does it reconnect if you reboot the laptop after waking up?

Put your 5GHz back on a/n with 20/40 coexistance.

Also, search through your keychain and make sure you don't have any dupliate key entries for the WIFI networks. The password keys for it need to be listed as system and not login in keychain to have really good reliability for reconnectivity.

There's two final possibilities.

1) There is an issue with high-speed n band chipset between the Mac OSX hardware/driver and your wireless router.

2) There is a region issue with the Mac OSX driver and your wireless router.

What country are you in? If it's the US, then there should be no issues as Apple tends to predominantly sell in the US, you'd think there would be no issues like this. If you are in another country, then it could be a driver glitch on the Mac side of things. Only way to rule that out would be with testing this against another n based router.

Don't know if it's available to you, but the Netgear WNDR4500 routers are really good and there's no issues with 802.11n at 450Mbps on them. I was previously running a WNDR3700 and upgraded to this one because I wanted to have TimeMachine backup capabilities with a USB hard drive connected to it and found a really good deal on one. WIFI is faster than my previous n based router and TimeMachine backups also work now. If you had to upgrade to something else, maybe look for something like this that can give you additional capabilities to make the "upgrade" a bit more meaningful.

You can call AppleCare if you have a support contract with them and they can help you. That number is on their support webpage.

Ironically, there are a few other people on here with similar issues lately, so it very well may be an OS issue or something. I find it weird that some people have this problem and others do not. Both you and the other user having these problems are outside the US. Maybe there's issues with the region code and reconnections on the 5GHz band in Mountain Lion or something.

Well, now supposedly (according to another thread) these kinds of problems are an OS issue. Supposedly Apple knows about it and is working on a fix in 10.8.3 whenever that'll be out. For now, I'd say just connect to the 2.4GHz network and deal with it. If your Internet speed is less than the speed of your 2.4GHz WIFI network, you're not bottlenecking anyway, so who cares until the update comes out. At least that your no-cost solution for now.

I hear you. Plug into ethernet and do your intial backup snapshot (that's what takes the longest). The subsequent differential backups that happens don't take that long to do over regular WIFI. Try that. That's what I did to speed things along and I was at 450Mbps N speeds and it still wanted to take 6 hours to do the backup.