Luche, I had same problem with my system. Urs and mine viturally the same. I have a 4800+ X2 AMD and a 9800GTX. I however found a bug using the AMD processor driver. Uninstall the AMD processor driver. Restart the computer and let it load the default microsoft drivers. Makesure it did do that by check the processor driver in the device manager. I believe the driver date should be 4/something/04 and I believe driver version is 1.0.0.0. When I did that, I uncecked compatability mode for AH and now I running dual core with no issues using the microsoft AMD driver instead of AMD's processor driver.
And it really works.
Stutter almost gone completely.
Still some FR issues, but they seem to be not that big, additional testing necessary.
But only as long as the program runs at all. Not running AH 2.14 in compatibility mode means frequent crash, particularly when minimizing the game - ye olde dual core bug.
Running in compatibility mode = stutter again.
In other words - I can now chose between stutter & crash.

But a big

to AKDogg