Something like this happened to me last year. I think that I needed a new fuel pump governor and fuel pump and plus I threw in a tune up, meaning new plugs, fluids, etc.

I think my tech charged me $300 for everything, but the pros would be able to diagnose it better than me.

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'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)