Ignition Coils Failure problems of the 2005 Mazda MPV

We own a 2005 Mazda Mpv. Four years after purchasing, the vehicle started to shake continuously and the check engine light turned on. Also when going uphill it almost can' t make it to the top. After taking it to the mechanic it turns out the engine was misfiring due a bad ignition coil. We replaced the coil. A few weeks later it started doing it again. This time we changed the six ignition coils. A few weeks later it started again with the same problem but not as bad as it was before. We kept using the vehicle like that, for a short period of time. Now the problem was worse, the vehicle started shaking and it started holding back. We took it to the mechanic again. He changed one ignition coil. It worked for a few days. Then it started shaking again. The mechanic change another one, the six spark plugs and one injector. It kept with the same problem. So the mechanic suggested to change the six ignition coils, and we did. The next day the check engine turned on again. I really don't know what is wrong and spend a great amount of money on trying to get the vehicle fixed. I have changed the ignition coils and still the vehicle is not working properly. After research online, we found out we are not the only ones dealing with this problem.