Could be a regulator gone duff( you're gauge might be innacurate), so might as well try seeing if you can up the pressure by adjusting it.
I would first though CLAMP the return line, this will ensure you get all the pump pressure, see if it revs ok, don't keep clamp on, otherwise it will be VERY DANGEROUS.
WHEN WAS THE FILTER LAST CHANGED ?
As said pressure with vac on is 3 bar vac off around 3.5 but up to 3.7/3.8 depending on how good your engine is!!
A rewire of the fuel pump is also a good idea, whether it is the pump or not.
tabetha