well, then, put a meter on the fuel rail, and see if it's holding pressure (it should fluctuate with inlet manifold vacuum, but stay pretty high).
If not, then tee off before the regulator to see how it's doing (this is a major pita- mitsu has a kit, but I doubt mere mortals could get their hands on it)- if that point holds steady (high) pressure, it's your regulator. If not, it's the pump. Put a meter on the pump power terminals- sometimes the relay will only work intermittently...

hth,
Toby

'89 V6 4dr- '4- Doris'