Here are some great pics of well designed suspentions.
Notice three things:
1. The steering joints are lined up with the a-arm joints when the steering is centered.
2. The steering links are the same length as the lower a-arm
3. The steering links are parallel to the lower a-arm.
I took a look at some of the pics you posted durring your build and i think that these are your problems. If any of these things are out of alignment, then you will get bump steer. If they are all off then you can get severe bump steer and this can make the cart wonder, or jerk to one side or the other when the suspension is compressed.
1979 Taylor Dunn 7" Lift, 26" Tires, 440 Rotax, Rear Disc Brakes, Air Shocks, Custom Roll Cage and Light Bar
Still under construction with more to come