I have an ASUS with the same problem, and an ASUS RT-N66U router. So I bought an ASUS USB-N53 Dual-band N600 dongle.
A complete disaster as unless I made some stupid mistake the drivers I downloaded for it didn't get on well at all with my laptop on Win 8.1. The "connected speed" was fine, but the throughput all over the place. Clearly visible on NetWorx graph, stuttering YouTube, slow-loading pages and terrible speed tests.
Back in its box and now on 72Mbps at 2.4GHz. Samsung SIII and iPad AIr brilliant on 5GHz, so it isn't the router.
My broadband basic info/help site - www.robertos.me.uk
| Domains,site and mail hosting - Tsohost
Connection - Plusnet
UnLim Fibre (FTTC). Sync ~ 56.6/14.1Mbps @ 600m. - BQM
"Where talent is a dwarf, self-esteem is a giant."
- Jean-Antoine Petit-Senn.
Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.