Handling/Steering Issues

[FeelGoodUnInc]

2003 3.0L X5
20" Staggered Wheels/Toyo Proxes

ok this is the problem i'm having, when im driving through some uneven roads...even the slightest my steering wheel just jolts left and right severely, a bit too out of control, i understand it may be due to the 20" setup w. the low profile tires but it's hard to believe that the 4.6 or 4.8 model has this same severe issue, i've never driven a 4.6 or 4.8 model but i doubt bmw would release a car stock that has severe handling/steering wheel issues that i'm facing, so my question is how can i improve this even ever so slightly? the 4.6/4.8 has better suspension im guessing as well as brakes, do i need to upgrade that on my 3.0 as well? or will a sway bar add-on make any difference in this field?

looking forward to your replies, thanks for your time to read this!