If it were me I would check the Fuel Injectors and Coil Packs.

You can check the fuel injectors by using a screwdriver. Hold the tip of the screwdriver on the injector and hold the plastic handle to your ear. You should hear a clicking sound as the injector fires. Of course a stethescope would work better. If one or more of the injectors is not firing then that's a problem.

To check the coil packs you can disconnect each one, one at a time and put some heavy gauge wire inside it and see if it will spark to the engine block. Of course a spark plug tester would work better. If one or more coil packs are not firing then that's a problem.

Also make sure that the rubber boot is good on all of the coil packs or you will get arching from the coil pack directly to the block, bypassing the plug. you can also check for arching at night when you should be able to see it visually. You should also be able to spray a fine mist of water without seeing any sparks around the coil packs.

If all of your coil packs are good and all of your fuel injectors are good, then you need to test compression and or perform a leak down test.

Let us know how it turns out.

Cory