Get a fuel pressure gauge and observe the pressure when it occurs. You have a Schrader valve on the fuel rail which makes it easy to attach a gauge.
Get thee to Harbor Freight and buy the kit. Pressure s/b 50 psi+.
Ed in San Jose '97 540i 6 speed aspensilber over aubergine leather. Build date 3/97. Golden Gate Chapter BMW CCA Nr 62319.