I'm in Crawley mate, my SH isn't rebooting, network was playing up a few weeks back but nothing to cause a reboot.
Have you done a scan using
inSSIDer installed/run on a wireless capable device/laptop to see what channels each network within reach of you uses then setting your SH to run using a channel not use? The firmware bug has been fixed but its worth a try if you haven't already. Multiple networks using the same channel would cause issues. The SH is in a league of its own so you need to put your arm round it
Modem mode is fine, should be anyway. If not then your SH is shot and needs replacing as your power levels seem fine. Note power levels seem to increase when in modem mode, if they jump to high and its unstable then you'll need a tech round to fit an attenuator unless you wish to buy one off ebay.
do you max out on speedtests via a wired connection?