I patiently wait and then flash these firmwares as they are released. I'm still seeing all the port forwards I enter drop after some period of time (usually a few hours to a couple of days). Disabling then re-enabling port forwarding corrects it for a time. I've got several servers running behind this thing and spent a lot of money on it. You can't beat the performance, but the port forward issue is killing me. It appears they're never going to fix or address this. I'm really disappointed about this. Multiple tech support requests have either resulted in them sending me on a wild goose chase or acknowledging there is a firmware problem and saying it's being worked on, but it never seems to get addressed...
Have you tried the DDWRT?
DDWRT New Version just posted
ftp://ftp.dd-wrt.com/betas/2015/03-27-2015-r26587/asus-rt-ac87u/
==> 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: OpenSSL memory leak + 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
CC