I was just posting this in another thread (sorry for the double post, but it might help):
Well I was fighting with a similar problem recently with the connection via 5GHz .
Firstly it didn't see the 5GHz wifi at all (my main notebook and cellphone without problems) - I found out that it doesn't work when the channel is set to 149 or higher numbers (router (TPlink N750) was set to Auto and the channel was 153). After changing this, the S3 could connect, but the speeds were incredibly slow - webpages were loading for couple of minutes or didn't load at all. I found that it acts that way if the bandwith is set to 40MHz, after manually changing it to 20MHz the speed really improved (11a - 23/23 Mbps; 11n - 80/90). In the case of 11n it is very close to the cable connection. The 2.4GHz worked well all the time - everything set to auto, the speeds were bit slower than in the case of 5GHz.
So maybe you can try to fiddle with the router settings as well?
I didn't try to change the drivers to older ones, so I don't have to repair it after every upgrade, but some people reported that it helped them.
Does anyone have an explanations why my S3 acts that way when connected to 5GHz wifi? Any suggestions? At all the settings tested, even when the S3 didn't connect or was very slow, my computer and cellphone didn't have any problems...