Hmm. Never tested a relay for resistance before. I'd suspect the switch side (85& 86) would be a pretty small resistance, but the load side would be infinite, at least for a standard always-off SPST version.
The FPR does vary the fuel pressure based on intake vacuum - but your problem seems like a flow issue. That pump seems a bit weak regardless...IIRC 43.5psi (3bar) is minimum system pressure for the M42.
Put a hose on the fuel pump and run it to a gas can. With the key on and the fuel relay socket jumped (or 12VDC on the fuel pump terminals), Bentley says you should see nearly a quart every 30 seconds. That's be a gallon every 2 minutes. With this test you can eliminate the entire supply system and isolate the fuel pump for testing.
If this test succeeds, then maybe a blockage in the system? Crimped fuel supply or return hard line, internally collapsed hose, blocked filter, plugged pump sock, something like that?
