2001 Mazda bucking on highway
 

Help! Just bought 2001 Mazda Millennia with 60 thousand miles. Car was not in use for some months. Drive home was 90 miles, uneventful. Then some days later, car started bucking on the highway. I would put my foot on the gas pedal, ease off and the car would not cruise, just slowed down. Then when I put my foot on the pedal, it would hesitate and then buck forward. This alternation goes on, rendering it almost un-drivable. No check engine light. Then I pull over, put my foot on the brake, the trouble disappears. Also does a "pop" forward here and there when on the road. Also when I start the car and it is still in park. I have owned the car a week and a half. It has done this twice. Mechanic shrugs. Has no idea. Mazda illiterate. Any help from you guys?