I am running 4x R9 290x (HIS and Sapphire) - usually get about 3.4Mh on regular scrypt - getting 1.4Mh on scrypt-n
My changes to 'standard' settings are that I am running a TC 25051 and I-18 (usually run 28632 and I-20)
Having some issues with black screens and the rig just locking up - but when it runs it is pretty stable and stays up for most of the day (only had it running about 3 days now)
I had a few stability issues aswell with R9 290's, I found the ideal (for stability), well for me anyway! I-19 and TC @ 24550. This nets me a conisistant 1.288Mh/s over three cards with good temps and 24/7 running.
Initially with I-20 I was getting lockups after 5-6 hours and black screens like you, the above seemed to be my sweet spot for scrypt-n.