You have to narrow the temp.range on cpu core.....single stage around -20C to -15C or probly -50C evaporator temp.
Cascade will yield -50 to -68 cpu core temp.
Dry ice temp. range from -30 to -45C.
Most San Diegos will run ok with single stage/dry ice @ high htt, but it will stuck probly around 240 htt with cascade.
Venus/opteron still a mystery with cascade, but for my 150 cabne0528 theres no problem with cld bug @-94C evap. or -50/60ish cpu core temp. Probly the memory controller was set differently...
Ricky's tweak or ACPI Multiprocessor tweak will help tame the cold bug in most SD/Venus, but it will put a tremendous heat load to your cooling. Especially with high vcore.