Could be a vacuum issue as the transmission in these cars depend on vacuum for shift quality (soft/firm), or a Bowden cable adjustment which controls shift timing (what RPM it shifts at). Or it could be a combination of both.

Try this link Diesel Vacuum system adjustments and do a search Bowden cable adjustment for info.

Hope this helps

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)