High data throughput, but constant disconnections in the 5 GHz band

I have the AC87U purchased to operate it as an access point. If you look at the reviews of trade magazines this is currently the best router on the market. Unfortunately, I had less luck with that.

First the positive:
the router has a very good wireless coverage, he reached the far corner of the house.
many settings, including operating as an access point.
Very good data throughput, I reached in the 2 GHz band constant transfer rates of 5 Mega bytes / s at about 10m away and a brick wall and a wooden wall. The remote station I took a Asus AC WLAN stick. In the 5 GHz band I reached 15 Mega Byte / s normally, between also 25-30 MB / s were possible. I tested the speed by I copied files to a Seagate NAS. In the 5 GHz band but also started the problems.

The negatives:
constant disconnections in the 5 GHz band. After about 15-20 minutes the 5 GHz network was automatically disabled by the router. While the LED on the device was still shining, but the 5 GHz network could no longer be found. Suspected first WLAN Stick had a defect, but other dual-band enabled devices (Netgear stick and I-pad) could not find the network.

The Merlin firmware did not solve the problems either.
Without functioning 5GHz band it does not make sense for me. To send large files from the PC to the NAS the 2 GHz band is too slow.