1993 Ford Probe

I just bought a 1993 Ford probe GT about 3 months ago. when i test drove the car, it was a perfect ride. on my way home the car just starts to jerk real bad. the person i bought it from said the motor had just been swapped out. every single gear from about 1.5k - 5k rpms, the car jerks uncontrollably. almost like it surges upon acceleration. i've change the klo2 sensor with a whole new air intake and that didnt seem to work. i've spent every single day for 3 months trying to figure this thing out, every time i end up empty handed. when the car is in neutral and you rev it up, its almost like it studders and sticks at 3k rpms all the way up to 5k rpms. please any help would be great!! thank you!! the motor has less than 5k miles on it