My money is still on one of the coils. I can not believe you replaced a fuel pump that had to cost $200 plus and was a pain to replace. 40 psi was less than 10% low which is an amount the computer could easily compensate for with little to no effect on how the car ran.
Coils on the other hand will make a car do exactly what your original post described and are only 31.99 each for a total of 95.97 at Advance auto. Order them online, enter promo code P20 and pick them up in the store. Total after discount will be $76.78 plus tax and you can change them in less than 30 min probably about 15.
When you get it fixed will you post back and let us know what was wrong

 

P0300 is rarely the ignition and it is usually caused by fuel delivery. You already replaced one major component and no joy so before you throw more money at the car test a few things first. The FPR is pretty easy to test. Get a fuel pressure tester and put it on the rail, start the engine, using a vacuum pump apply vacuum to the FPR and see if the pressure at the tester changes. If I remember right when you are at idle with high vacuum you open the FPR to allow fuel to return to the tank and when you are running hard (which requires more fuel pressure) you have less vacuum so the regulator closes and you increase pressure. SO by applying the vacuum with the pump you should lower the pressure in the rail. If that tests out good then see about noid testing your injectors and also consider flow testing them at an injector specialty shop. We have one here that will clean, flow test and rebuild them for about $30 a pop, which beats the hell out of the $60 per pop new ones are.

After that if you still have issues then I would look into the coils. Or you could swap one coil for the other and see if the cylinder that has the most missfires (in this case #2) moves to where the new position. If it does then look at the coils. That test does not cost anything but some time and a little fuel.

As for the fuel level droping fast from 1/2 to 1/4, I would not worry too much about that my gauge does that too. I think it is how the arm moves in the tank, it makes you think you have more than you do. Mine will hold on full for about 100 miles or so but from 1/2 to 1/4 is only about 20- 30 miles at times. I gauge my economy on a full tank not a partial tank and I get about 250 miles from full to 1/4 or about 24 combined MPG.

Massey

 

adding to massey's advise,,, check the wires and connectors to the #2 coil and the #2 injector. look for any signs of damage.

 

I'm still holding out for getting that mis-matched replacement fuel injector out of the equation first.

 

its like 30 bucks to get them all cleaned and flow tested to make sure they are flowing the same. i had my new ones tested and adjusted.

 

Massey I am sorry but I have to respectfully disagree in this case. You have made a lot of very useful post here (your what 3800 fwd engines work post is excellent) but I just think you are missing it completely here. My personal experience was coils. I did a search for P0300 and found this:
OBD-II Trouble Code: P0300 Random/Multiple Cylinder Misfire Detected

Notice coils come before fuel.

Want to make two $1 side bets here. I will bet you $1 that it is not the fuel pressure regulator and bet you another $1 that it is a coil or coils. I will paypal the money if I am wrong.

 

I am guessing that the FPR is good too but the test only takes a few minutes and you can rule it out. Might as well.

I am not saying that coils cannot cause this but in my 20+ years of turning wrenches on things I have found that the p0300 is usually not the ignition system and is usually fuel. I have actually found the P0300 also to be caused by a bad MAF, MAP and even an O2 sensor. So I am not ruling anything out here and this is my least favorite problem to track down. Just too many things that can cause it.

Massey

 

It's interesting how that site down-plays an injector as a possibility. If one injector is putting out more than the other two in that bank for instance, that whole bank gets leaned out. Meaning the two cylinders that should be flowing correctly are now running leaner. I've been through this very thing once (PO300 because of an injector), but mine was self-inflicted from putting in an aftermarket (different brand) injector.

 

69 I am not dismissing you thought completely I just consider it less likely based on:
1) I have seen the coil packs from 3 Camaros and 2 out of three coils had failed on all three.
2) The fuel injector was replaced and nothing changed.

If you have a 4th gen 3.8 are are your coils original oem?

 

Also When I start tracking down a problem of this sort I go at it from both a how likely is the diagnosis and how much to confirm the diagnosis direction. In this case it would seem the cheapest likely possibility to rule out would be coils infact I would say that before the fuel pump.