fuel pressure dropping over time

diymirage said
The sending unit is new, and really, just a tube to suck fuel through

The gauge is not, but correspondes quite well with the mechanical gauge under the hood

I meant the electrical sending unit for the electric pressure gauge. But you said the mechanical gauge reads the same as the electric one in the car?

I'd maybe bypass the regulator all together if you have the fittings to try it out.