Help

I have a 97 pontiac grand am, the check engine light comes on and off, as well as the coolant light. Sometimes the car does not start, or it will start stall and seem like its not getting any gas. Recently the car started making a grinding noise when you turn the wheel to the left and a squeeking sound as if a belt is wet. We had it hooked to a computer and only said multiple misfires. Replaced the oxygen sensor, spark plugs, new brakes. What can I do to find out whats wrong since the computer wont tell me anything?