I'd bet that there is either a vacuum leak causing the tranny to shift late or it is set up wrong to begin with.
My 240 shifts really sweet (contol rod based shift controls) and the 300 has been a struggle to find just the right combination of vacuum, bleed valve adjustment and modulator adjustment to make the shifts consistant all the way through the power cycle.

Slipping is when you get concerned... A hard shift should be correctable.

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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