Intermittent function is also a common symptom of impending fuel pump failure.
When not working, you can check for voltage at the fuel pump with a test light. I say a light, because digital voltmeters usually do not register fast enough to detect voltage during the short interval that the pump(s) fire when the ignition goes to the On position.

An alternate method is to provide 12v directly to the fuel pump, and see if it spins.

Intermittent fuel pumps will generally start if given a firm whack.
While you may not actually have a 124 fuel pump on your custom rig, the above holds true for most fuel pumps.

Best of luck with your winged wonder.

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1986 300E 5-Speed 240k mi.