Have you tried 378.52_alpha3? My 5GHz band has been fine but It has also never been an issue for me. In fact, the 2.4GHz band has been much more problematic for me than the 5GHz band.
So I gave DD-WRT version r26446 a 4 day trial and it kept dropping the 2.4GHz band altogether after 18-24 hours of uptime. I tried a daily reboot at 5:30am, a nice feature Brainslayer has built into DD-WRT's GUI, in order to see wther or not a fresh boot each day would resolve this issue (it had absolutely no effect AFAICT). The only way I could "regain" the 2.4GHz band was to do a factory reset and MANUALLY reenter all of my configuration settings (no fun son).
Upon the 2.4GHz band failing for the 3rd time in as many days I went back to Merlin. While 378.52_alpha3 has been a little wonky from time to time my solution has sorted it out. That is to use the WPS button method to erase the nvram (please search the FAQ and this forum before asking how to do so), put the router into recovery mode and then reflash the router using the Asus firmware recovery utility.
Of course, I did a backup of the router's configuration settings with the fantastic utility that John9527 has been kind enough to make available (we're not worthy). This way, after reflashing the router, a few keystrokes restores your settings with the exception of any closed-source stuff. So you still have to manually reenter those settings but that is a piece of cake as compared to having to manually reenter all of your settings as we had to do before the wonderful nvram save & restore utility from John9527.
Since 2446 here are the fixes to date:
==> 26622 --- 2015/03/31 -------------------------------------------------------------------------------
[26618-26619] Update: Kernel 4.0-rc6
[26588-26590,26595-26601] Fix: Remove unnecessary code for hostapd
[26602-26603] Fix: SSL buffer implementation
[26604-26615] Fix: Network interface label
[26616] Fix: IPv6/radvd
http://svn.dd-wrt.com/ticket/4022
==> 26587 --- 2015/03/27 -------------------------------------------------------------------------------
[26514-26515,26559-26560,26587] Update: Kernel 3.10.73, 3.18.10, 3.19.3, 4.0-rc5
[26491-26494] Update: OpenSSL 1.0.2a
[26525-26526] Update: DNSMasq 2.73rc1
http://svn.dd-wrt.com/ticket/3847
[26528-26529] Update: libpcap 1.8.0
[26527] Update: BusyBox 1.23.2
[26519-26524] Fix: [Broadcom] Timer issue
[26538-26539] Fix: NAT rules broken for PPTP/PPPoE dual WAN connection
http://svn.dd-wrt.com/ticket/4033
[26564-26586] Fix: SSL memory leaks + Enhancements
[26508-26510] Add: Network stack offload using multiple cores
[26511-26513] Add: PPTP acceleration
[26534-26535] Add: DIR-860L A1 support
[26536-26537] Add: New chipset support
[26530] Add: php5 curl support
[26558] Add: Display VHT capabilities if available
[26543-26556,26562-26563] Add: DIR-859 A1 and QCA956x support
==> 26490 --- 2015/03/19 -------------------------------------------------------------------------------
[26452,26474-26476,26488] Update: Kernel 3.2.68, 3.10.72, 3.18.9, 3.19.2, 4.0-rc4
[26477-26479] Update: Quagga 0.99.24
[26448-26449] Fix: VAPs on triple radio
[26450-26451] SemiFix: Repeater Bridge broken for k2.6 and k3.x
http://svn.dd-wrt.com/ticket/4038
[26469,26484-26485] SemiFix: PPTP
http://svn.dd-wrt.com/ticket/3280
[26467-26473,26489-26490] Fix: L2TP
http://svn.dd-wrt.com/ticket/4043
[26462-26463] Add: L2TP MPPE encryption support
[26480-26483] Add: User customizable config for wireless security
[26486-26487] Add: Octeon MMC support
[26464] Add: WR740NDv5 support
http://svn.dd-wrt.com/ticket/4035
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