Steering Rack And Pinion problems of the 1996 Nissan Altima

I was driving onto the interstate, and I was just about off the on-ramp and I hit a bump. My car's steering lost control and it spun around twice and I nearly caused a major accident on the interstate. Thank god I gained control. I changed my tires the next day and the steering still feels very loose. Everytime I hit the smallest bump, the car feels unstable. I aligned the whells for $80 and I am still having the problem. My mechanic says everything looks fine. Please help. I never know what will happen when I drive. It all feels very loose like there is something wrong with the connection between the steering and the front wheels. In the rain I am especially scared becuse it makes it 10 worse.