Pull the spare tire down and take a look at the fuel pump module after loosening it from the frame. If the back is badly corroded replace it with new one. It takes about a 1/2 hour of your time to find this problem, if it exists, if not then you can rule out the fuel pump module probably. Next swap the fuel pump relay with a good one. As the pump runs at different speeds it may be not functioning properly due to low voltage.

 

Before and after. They just keep swapping cops and plugs and it runs a bit better for a little. Would a bad injector or stuck lifter cause all these?

 

I've replaced the fuel pump module already. How difficult is the relay swap?

 

Well I swapped the injector and still #3 misfire

 

I had a problem that sounds just like yours and it was a bad maf sensor. Change that and see what happens.