While driving the speed goes up and down on its own the automatic transmission of my 2008 mazda 3 shifts down from the 5th to the 3rd gear. this causes the car to jerk violently and the controls to illuminate including check engine, no traction and at. my mechanic could not find a cause of this, and he serviced the transmission by changing the filter and transmission fluid but the problem persists. one mechanic said it is the speed sensor another the transmissions computer i have spend about 1,800 and i still have the same problems. it is very scary for my kids and myself while driving and i can't afford to get another car. what is the problems with mazda with my car? can someone please help?! also all the electrical stop working about two years ago, another thing is the steering wheels problems when making a turn it really time forever and it is really slow and heavy.