saucer thing only gets 12v with thottle open, its resposible for firing the 5th injector briefly when you put load on engine i.e. boot it. but the 5th inj only gets an earth path when cold. Perhaps the loom is shorting out for this, pull the plug on the 5th injector see if it improves. I had mine unplugged for a week and it made no difference to how mine ran fregardless of stone cold or boiling hot!
edit: also if you have a spray bottle put a light misting of water over the ign components, check it at night like danny says, look for arcing.