fuel pressure dropping

have a northern imca sportmod and having trouble with the fuel pressure dropping dowm to 4lbs. car runs like your on the rev limiter but tacking onlly around 6000 rpm. Replaced the fuel line thinking the suction side may be collapsing,not it then changed the fuel pmp ,no change. When first started will read 8lbs on a carter pmp but then it will start dropping. I would say it was a ignition break down if it wasnt for the fuel pressure dropping.any ideas, thanks

Drops to 4psi at RPM? or at idle?

Cell vent plugged?
Pick up tube?
Broken or cracked fittings?
Fuel filter plugged?
Pump wore out or leaking valves?
Regulator?
Faulty pressure guage? I'd look at this before doing anything else.