I have noticed this same thing when driving down to the Bay Area with my 240 then my 81SD and a couple of months back with the 84SD. I was kind of thinking maybe the air fuel mixture (ALDA) is a little out of adjustment for the higher elevations.
I am only at 3100 feet so the adjustments would probably be pretty minor.

I will probably mess with the 84SD this weekend as I have a bunch of maintenece to do. I'll let you know if It makes a difference.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique