my '79 300sd always cranks and starts when cold, but occasionally, when hot, she not only won't start, but won't crank at all, i turn the key and nothing happens, though the dash lights come on (the glow just flickers or doesn't come on). if i open the hood and then shut it, causing vibration, it will sometimes start again, but sometimes i have to wait a half hour or so until she decides to crank again. then again, sometimes she will go for weeks without this problem, so it's hard to diagnose when it only happens occasionally.

any ideas?

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1979 300SD 190k Blue/Parchment