Ok ill try whit official fw to see if it helps, anyway i need Merlin becouse i need to acetive jffs to run custom scripts becouse my ISP use vlans 6, 2 & 3 for Internet, IPTV & VoIP.You can try and disable beamforming. If that does not work, flash the latest Asus 3.0.0.4.380.1354 firmware, then reset the router to defaults. I have had better results with the Asus firmware over the Merlin firmware on the RT-AC88U. I am running the 1681 beta release on mine. Here is pretty good guide for setting up Asus routers....
http://www.rickygao.com/tuning-the-asus-wireless-router-to-best-performance/
Where can i get the 1681 beta??