300 SDL shifting problem

[The Toecutter]

I own a Mercedes 300 SDL. Recently, it began to shift at a much higher RPM than normal. Normally, it shifts up at 2500 rpm or so from 1st to 2nd gear; now, it tends to slip shifting from 1st gear to 2nd gear. At 30% throttle(without even hitting the throttle hard enough to normally activate the kickdown lever), it won't upshift into 2nd gear until ~4,500 rpm. When 2nd gear is finally engaged in the transmission, there is a noticable but brief surge of acceleration, then more slipping if the accelerator is pressed too hard.

A transmission shop took the transmission out of the car(under warranty), ran a few tests on it, and could not find any errors(they may be lying about that). Any indication as to what could be the cause of the problem?

If the transmission is not an issue, perhaps this is a vacuum issue?