You may have a failing camshaft position sensor which is internal to the distributor. You will need to disassemble the distributor to replace it if this is the concern. Here is the explanation of how the Camshaft Position Sensor works. If the sensor is shorted it may only fire some injectors.
PURPOSE The Camshaft Position (CMP ) sensor utilizes a hall effect sensor to provide crankshaft position, base timing and engine speed information to the Ignition Control Module (ICM ). The ICM supplies this information to the Powertrain Control Module (PCM ) as the Profile Ignition Pickup (PIP ) signal. The PCM uses this information for fuel injector timing and calculating ignition timing advance and dwell.CONSTRUCTION The CMP sensor is located within the distributor and is comprised of a hall effect switch and a camshaft driven 8 bladed shutter plate.OPERATION Each blade on the shutter plate corresponds to a separate position on the crankshaft. As the shutter plate passes through the hall effect switch the output of the sensor alternates from 0.0 to 12.0 volts in a square wave pattern. This output is sent to the ICM where it is converted to the PIP signal.