This is what it says on their support page:
Option 3, "Crash Reset"
This will remove the current firmware. Next time you try to login, a message will show up, asking your for a firmware file/image. These can be downloaded from ftp.dlink.co.uk.
Power OFF the device. Press and hold the 'reset' button; while doing so, power on the device again and keep holding the 'reset' button for another 10 seconds. Then release the button and wait until device restarts.
If it works could you please tell me because I am tempted to try out some different firmwares but don't want to incase it makes my problem o the router restarting every couple of hours worse.