Speed Sensor problems of the 2008 Mazda MAZDA3

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.

Since the course of about a year, I have experienced traumatic moments while driving my vehicle pertaining to it's electrical/transmission functionality. When driving the vehicle at normal speeds (ex. 50mph) the Mazda would buckle, rev abnormally, all while my gear indicator would flicker. This would happen after about 10 minutes of driving and continue on for the remainder of the drive about every 20 minute interval. This is quite unsafe, and the repairs that multiple mechanics including Mazda themselves suggested to repair the issue did not accomplish anything unfortunately. Repairmen flushed and replaced the automatic transmission fluid, replaced my output speed sensor, changed out my shift solenoids, gear shifter, and even replaced my entire pcm. I have not only taken pictures and videos of these horrid incidences, but there seems to be other instances of people having this identical issue with their Mazda's as well. If there is a way to show my video examples for study, then I will gladly send them.