Engine shudders randomly

I have a Mazada Protege, 12 years old with only 55k miles on it. The shuddering is random. It happens at odd times, with no warning. Sometimes when I have just started the car- the engine will inexplicably struggle and then fail. Moments after that, I will start the car right up again and, while the engine light will come on, the car runs just fine. I give it a day and the light goes back off on its own!

Also, I've had trouble with being at stop lights and accelerating, slowing down and then hitting the gas pedal, or going up a hill and accelerating. It seems like it's just not engaging and the shuddering will start and then just as suddenly, the engine will accelerate and the shuddering is gone! I wasn't driving the car much for a few years, but the last few months I've been driving it daily. It hasn't seemed to make a difference in the engine problems either way. I can go weeks with no problems and then have problems several days in a row. I've had the engine checked, filters replaced, timing checked, everything. It still shudders and my, usually trustworthy, mechanic hasn't come up with any solutions.