What causes the car to understeer and oversteer

What year and model car do you have?

The 3 steering box mounting bolts tend to loosen up over time especially if its been replaced. I've seen it multiple times.

If the car was aligned using the factory specs it will tend to be unstable. I like 2* positive caster, .5* neg camber and about 1/16" toe-in. You typically cannot get enough caster using stock components so I replace the UCA bushings with Moog Problem Solver eccentric bushings.

My 68 with 215-60x15 front tires with those alignment specs tracks like a freight train with no hands on the wheel. You must tell the alignment tech what specs to use though.