My wireless connection appears to be dropping out all the time. I have checked on two different laptops, one that connects at 300mbps and the other at 100mbps, so it's not a range issue.
I've ran a ping over the course of around 8 hours, and it will drop off constantly, then reconnect. Just came from a crappy thomson VDSL router which had no such issue. My wife is now nagging me to get rid of this draytek

I've disabled QOS, tried factory reset to no avail. Running 3.6.4 firmware

I've noticed if i initiate a big download on my laptop using wireless (i.e download a game on steam) it will start downloading very quickly at 5MB /s then gradually slow down to 400 bytes per second, then the connection will just be lost and i won't be able to browse the internet. I have to disable wifi then reconnect.

Very frustrating for a £200+ business class product

HI
Have not notice any issues with mine or other complains here in the forum for this model

First I would say go to Wireless LAN >> Access Point Discovery (AP) and scan to see what channels are near you
Use a channel that is empty

Turn the 2 side antennas slightly on the sides (keep middle vertical)

Try another server for testing downloads ... when i was downloading from youtube, it was starting with high speed and did end up slow like your case but was the server (and was even wired)
A good test is here
http://www.thinkbroadband.com/download/

Other option is to disable WMM (is like QoS but for wifi) ... Wireless LAN >> WMM Configuration

If still not good all above try modem code 243601 or 243701 ... my own past tests showed me better wifi range with them 2 modem codes (just use the *all* file if is the same firmware (example 3.6.4 is fine changing modem codes using the *all* file)

go to wireless lan - advanced setting and set the channel bandwidth to 20MHz, that usually helps when I get packet loss.

Also try setting the wireless channel to auto or 3 or 9 (to avoid the most commonly used channels).