Q: My car is jerking back and forth at high speeds

The first thing that needs to be done is to connect a scanner to the car and retrieve the code. This information will lead a technician down a diagnostic path. There is an important distinction that needs to be made here. Does your car "jerk back and forth" while accelerating or at a cruise when the throttle and speed are held steady? These are different circumstances in regards to what could be wrong.

Surging at a cruise is usually a sign of a lean running condition caused by fuel system components. This could be the MAF, an injector, fuel pressure regulator, or fuel pump. Bucking and jerking while accelerating is usually do to ignition components. If there is a vacuum leak or the fuel pump can’t supply enough fuel, you can get similar symptoms. The problem could be a mixture of several of these.

As for why it doesn’t happen after twenty miles, this could be because a motor needs more fuel to run when it is cold. If the fuel system is performing as it should, this could easily be the cause. Several components have warm up cycles that may not be working correctly. If they warm up to slowly, numerous drivability issues can arise.

I recommend having your Check Engine Light diagnosed by a qualified technician, like one from YourMechanic, to help you figure this out.

Good luck!