I agree Mannyg, once they connect they're fine, no issues, but if you restart... Also, I have my 280x hashing around 2.6Mh on 14.6 beta drivers in BAMT.  I use the same settings for x11/x13/x15, the below conf is for my Sapphire Vapor-X 280x.

"intensity" : "16",
"worksize" : "256",
"thread-concurrency" : "8192",
"gpu-threads" : "2",
"gpu-engine" : "1080",
"gpu-memclock" : "1450",
"failover-only" : true,
"no-pool-disable" : true,
"api-network" : true,
"api-listen" : true,
"api-port" : "4028",
"gpu-powertune" : "0"

Sure wish Nicehash would chime in and let us know what is up with these connection issues!