It's likely the fuel pump relay, especially if it's after the car was running and heated up. This is a pretty standard problem for these engines. There is no easy DIY test for this that I know of, just replace the relay.

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Doug
'83 300SD "Old Gold" 170,000 mi.
'82 380SL "Green Machine" 190,000 mi.

Some days you're the pigeon, some days you're the statue... I'd rather be the pigeon!