Is the belt tensioner adjusted so that the indicator is in the correct range? Do you have a 124 Maintenance CD? If not, it's a great investment. See xhttp://skinnerbox.steaky.org/Service/W124/w124CD1/Program/Engine/119/13-3420.pdf (remove the x in front of the URL)

If the tensioner is not in the correct range, it's not providing enough (or too much) tension. You can crank the tension past the adjusters ability to function. Don't ask me how I know this ;-)

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Arctic white '90 300e 150K
Arctic white '92 400e 100K
'94 Suburban 4x4 diesel, 150K