1984 500SEC engine stalling

Folks,
I am a new Mercedes owner, never had one before, love it. It is a 1984 500SEC with only 150,000 miles on it. I have a problem with the engine stalling out at low speed, when I stop with the transmission in drive and sometimes cornering. I've had plugs, wires, etc replaced. My mechanic now tells me that it's because it's an engine designed for leaded gas with lubrication and that using unleaded gas has caused it to flood out and affects it at low speed but not at highway speed. Would anyone be familiar with this problem or have any other theories about what is causing it to stall out? Thanx!

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