Have you solved your problem yet? You never said what the cause was.
I don't have any stability problems except for a shimmy at moderate to high speeds. I attribute that to one of the wheels as rotating them increases or reduces the shimmy. Then again, maybe it is related...I have had the tires and wheels balanced three times since new, and the shimmy is still there...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car