The ROM chips definetely must be 40.68 and not 40.69. :-D
Second, try to press the 2 mouse buttons at the reboot to enter the Early Startup Menu. From there you can choose Original Chipset (OCS, ECS, Enhanced=AGA) or Disable CPU Caches. Try them with this game.
I don't think that Blizzard can cause problems. At least with me I haven't any problems with it. At the reboot you can disable the Blizzard by pressing the button "2" (If I remember correctly). Then your Amiga will have the onboard 020.
