Way back when my A1200 was untowered and I was using a Blizzard1230 I had to remove the trapdoor-cover and put spacers under the 1200 to get some air under it. If I didnt - it crashed after a while. I suspect that some A1200s - like mine, are more sensitive to heat because a friend of mine later bought that card from me and used it in his A1200 without need for removing trapdoor-cover or putting spacers under the A1200. That made me realize that my A1200 isnt exactly the best one manufactured and that the Blizzard1230 was very stable indeed.
/Patrik