Hey Dave!
Does it go away if you disable a core or 2?
Yup...indeed it does! I can either disable a core in BIOS, or boot with /MAXCPU=5 setting and either one works to product a stable non-spiked CPU behaviour.
This is what has made me think that the ACPI driver has an "issue" with the 6-core setup. Please note, when I say "issue" I do mean with my hardware combo. Now, if others pipe in and we see a trend, well, that'd be something different, which is part of the reason why I'm posing this question.
...Does changing any BIOS options such as "cool and quiet" help?...
I toyed around with numerous BIOS options in the past. For the most part I have all of these disabled because I find the ACPI PM scheduler NOT to work effectively, nor efficiently to start off with. Quite some time ago I shut off the ACPI PM and simply let the machinery run as it may, meaning: pedal to the metal, it's a full boost speed across all the cores. On my Phenom that means 4G when in 5 core mode and about 3.8G when in 6 core. Through experiments I found a single core that wasn't quite as tollerant of too large a boost, so I shut that core off in BIOS if I intend to run at the 4G speed. That used to be my default mode since the 6 core boot was a hit-n-miss, and eventually as ACPI drivers changed it actually caused a consistent TRAP, so I eventually gave up on it altogether.
Now that I am in the 6-core mode by default I may go back to re-test, but I suspect the previous brick-wall was really a CPU core specific hardware limit, and that's not going to be overcome by a different motherboard.