Car RPM drops on full throttle, no power

I have a 1988 Alfa Romeo 75 3.0 V6, with 75k miles. It’s been off the road for 5 years. Been working on the car for a while.

Taking the car for a test drive, it lacks power and the RPM drops when you apply full throttle, followed by a pop from the engine. There is no power, and the car feels generally unhealthy.
The idle is okay when warm, however sometimes (usually when cold) the idle becomes very low and sometimes cuts out.

Any help would be appreciated.

Sounds like it’s going lean.
Per Wiki, this is analog L-Jetronic fuel injection.
Possibly gummed up passages.

If its running lean, it could be gummed up fuel injectors, a blocked fuel filter or a faulty fuel pump… right?

Yep, any or all of the above.

Just managed to get a brand new fuel pump and filter fitted. Issue still not fixed

If it’s L-Jetronic FI, make sure the airflow vane moves smoothly, no hitches. A cylinder compression test makes sense at some point.

Does it show any error codes?

The issue you are experiencing can be caused by several things, including: bad fuel filter, bad air filter, bad TPS, bad EGR, etc…

Replacing parts at random hoping to hit it on the first try can be a costly experience. Not sure what the fuel pump replacement cost you, but could have been avoided with a 5 minutes test. Recommend you take it to a mechanic to have it analyzed.