I finally got my airport express (AE) to stop dropping the signal. It took me so long to finally getting it to work so I would like to share what i have done.
Apartment size is aprox 100 squared meters, the AE is set up in the middle of the apartment.
I got it the AE so I could have all of my network devices running in dual band running at 5Ghz (faster connection) and one iphone 4 at 2.4GhZ.
I have an Apple MC414Z/A AirPort Express Basisstation 802.11a/b/g/n running firmware version 7.6.4
Gateway set up
My set up includes a Gateway (router-modem combination) by my internet provider running on 192.168.0.1 with both WIFI and the DHCP disabled. The internet company uses a rotating IPV6 protocol. I have opened all the ports on the gateway that Itunes and airplay require to work on both UDP and TCP which detail. 1119, 6113, 1120, 80, 3724,443 and 8689. In addition I have allowed access to all the MAC addresses of all the devices that i have on my network, including the AE.
AE set up
An ethernet cable is connected to the first LAN port from the gateway to the AE's WAN port. The AE is set up to create a wireless network with no hidden network. The extra 5Ghz network though is also active so it broadcasts two ssids. The AE is configured to a static IP address 102.168.0.2, the router address is set to 192.168.0.1 subnet mask 255.255.255.0 The DNS server hasn't been a problem but i am currently using 42.182.18.228 DHCP only is enabled starting the DCHP range at 192.168.0.20 to 192.168.0.200 with a 1 day lease allowed. Radio mode is automatic and channel 1 and 36 are used for 5ghz and 2.4Ghz correspondingly.
Individual device set up
Every device in the network is set up with a static or manual IP address starting from 192.168.0.3 until 192.168.0.10 with the router on 192.168.0.1
IpV6 protocol is disabled in all of the devices. For more information on how to disable IpV6 on mavericks (http://help.unc.edu/help/how-do-i-disable-ipv6-on-mac-os-x/) The devices such as laptops and Mac mini and apple TVs are set up only to recognise the 5Ghz network, while the iphone 5s recognises both 5Ghz and 2.4 and an Iphone 4 only the 2.4ghz
Extra set up
I have connected an FM transmitter on the sound output of the AE (http://www.czhfm.com/index.php?main_page=product_info&products_id=16) using an analog cable. The range of the FM transmitter is such that it covers all the apartment and so i saved money on buying wifi speakers such as sons. The FM transmitter is set up at an empty frequency on 82Mhz.
Apple tv is set up with digital audio out put to a samsung sound bar. The samsung TV is not connected to the internet. I don't deem it necessary and I have read about problems with bandwidth hogging from the samsung tvs.
In conclusion this setup has allowed me to have music from my itunes library all over the house using the remote app on the iphone 5s, with good interconnectivity among all the devices (like it is intended) but it took me three months to figure it out. I believe the problem was the dhcp handling by the gateway and the IPv6 enabling. The AE handles DHCP now but its not really being used since all devices are set up manually and there are no clashes. The guest network is disabled since it has been reported to produce drops in signal as well.
i hope this helps