Are your WiFi devices actually dropping every 5 seconds, or are you just worried about WiFiman readings?
If it's just WiFiman, you seem to be extremely close to the router at -30 dBm, like touching it. Try backing off a little.
If devices are dropping, try plugging the ASUS into a different power outlet, or better yet, a UPS outlet if you have one. Try not using a power strip if you are plugging into one.
If somehow available, try a different power cord for the ASUS.
As a longer-shot, reflash the latest firmware again, or try Merlin Firmware if your ASUS model supports it.