If one end bounces faster, not one side put a softer spring on that end.

Mount the bottom of the shock on the inside hole like you did with the gray rear spring. The gray spring is stiffer than the silver spring. When you moved it to the inside hole it lowered the frequency. Now it appeared softer like the silver spring.

Where you locate your battery, speed controller, receiver, & transponder can affect the overall balance of your car. I have 2 identical batteries so my SC10 is consistent.

Try another spring on the front? Have you tried the blue or gold front spring?