1. Check that there is no excessive friction or slop in the suspension, dampers and wheel axles. It is also impartant that the diff is smooth.

2. Put the chassis to rest on support blocks (for example Hudy Chassis Droop Gauge Support Blocks) one just behind the motor and the other under the steering system.

3. Loosen the screws to the top deck, push down the chassis to the support blocks and tighten the screws to the top deck. Now the chassis is straight.

4. Measure that the damper springs are preloaded equally L to R with a 5 mm ride height. Now the corner weights L to R are about equal.

5. Put a 0.5 or 1 mm aluminium roll center spacer under both rear suspension blocks.

Now you have a good base to try different springs, damping and droop settings front and rear to find a good balance. If it still isn't handling well the problem should be the tyres.

I recommend to always use a spool with rubber tyres. You will loose some corner speed into tight corners but still use the spool until the car and driving is perfect then you can try the one way.