Did you ream the carbon out of the gp holes? You could have some carbon build-up in there that's touching the gp's and shorting them out, or you could have another bad plug or two. At those temps, my car will light right up as long as all five plugs are glowing good but get even one bad one, and it will significantly increase the cranking time and make it stumble on startup.
Check out Dieselgiant's
gp troubleshooting page for info on the 80 amp fuse and an all around fantastic gp guide.