W126 420SEL intermittent stalling

my dad's 420SEL is stalling intermittently... it's impossible to predict when it'll happen.. the engine cuts and power steering/brakes go...
if i try to start it again it'll just crank over and over with no results...
so i wait 20 seconds and it starts again
it's only happened a few times, but it's a dangerous problem (what if i stalled while turning right across traffic?) so would anyone be able to guide me in the right direction to get it fixed? my guess is the distributor, but seeing as it's intermittent it's hard to diagnose
anyone encountered a problem like this? any help is appreciated
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The fuel pump is powered by a relay (fuel pump relay) which acts like a big ON / OFF switch for the pump's electric motor. The car is "of the right age" for this old relay to become intermittent. You see the result.
Replace the fuel pump relay.
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Thanks for that, I'll get it looked into ASAP

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