Help! No one can figure out what is wrong with my car.

My check engine light goes on and off. Sometimes it flashes and sometimes it
stays on. My car shakes violently, especially when I am stopped at a red light.
The problem also gets worse when I turn on the air conditioning. When I press
the gas to accelerate, it goes slow and then all of a sudden shoots forward. I
have taken it in over four times and every time it is fixed for two weeks at the
most but then starts the problems again. Since the check engine was reading
as a cylinder four misfire, I have had all of those replaced. This final time I took
it in, he took everything apart, cleaned it, and put it back together. This was the
most helpful as my car ran fine for about two weeks but now the problem is
back. Someone please help me this is becoming a nightmare. He said I might
have to replace the computer but at this point I don't know if he knows what
he is doing.