As far back as I can see on Lucy (2002), these should stack. The resist from Aura of Heat is slot 4, Resist cold is slot 1 (was slot 2 briefly).
Also, I'm using the middle level potion (10 dose Potion of Heat), and I see something strange with my CR. It will go +15 immediately, as it should (according to
http://web.archive.org/web/200102150...dalfh/potions/), but then it drops to +10 after a tick or two. If I zone, it goes back to +15, permanently.