I believe that your issues are unrelated. On the first, either the cat, or the rear O2 sensor could be bad. Any performance problems once the car warms up?
The service stability system warning is usually caused by the steering position sensor. However, that generally has not activated the system. You'll likely have to take it to a dealer, or somewhere that has a Tech II or similar scanner to find out what the problem is. Besides the steering position sensor, a bad wheel speed sensor in one of the hubs, the lateral accelerometer, or yaw rate sensor could also cause the problem.
Black 2000 SSEi, Silverstar Headlights, Angel Eyes fogs, 3rd brake light overlay, hi-flo cat, drilled 180 degree thermostat, Intense FWI, PCM, shift kit, and 3.4 Pulley. Solid front mount.