Backfire, stuttering, no power with gas

This sounds like a possible couple issues. Starting with checking for fuel pressure when you are driving the vehicle. The fact that this runs ok except when you give it gas points to an air flow issue/restriction or a fuel supply issue. Basically the air fuel ratio needs to stay around 14:1 and it can do this when idling but when you give it more air by opening the throttle either the air can't get through the engine to the exhaust or the fuel can't keep up.

https://www.2carpros.com/articles/engine-has-low-power-output

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

https://youtu.be/HiI_eeFg-2A

Let's start with all these guides and work through this just to find out what appears to be out of line and then we can go from there.

Let us know what questions you have. Thanks