These transmissions do have the habit of skipping through the intermediate gears darn quickly. My '87 300TD does this to a degree every now and again.

I'd start by adjusting the bowden cable. It sounds as if it has stretched a bit. The cable should be just tight with the throttle closed - no slack, but no real tension either. The bowden cable controls how long the transmission holds each gear. Opening the throttle more pulls on the bowden cable, telling the transmission to hold each gear longer. Too much slack in the cable and you get early upshifts - the transmission doesn't think you have your foot on the gas so much.

Driving style can also cause these stacked shifts. If toe way into the throttle to get started from rest then back off as the turbo spools up you are encouraging the transmission to shift up. Less throttle = less demand for acceleration = time to shift into a higher gear.

The vacuum modulator adjusts shift quality - quick and firm vs. soft and sloppy. It doesn't affect shift timing. Assuming fluid levels are correct, it sounds as if a bit more firmness might be in order.

This car also has a system which softens the shifts until engine coolant temperature reaches 50C. It's controlled by a three way valve mounted on the drivers' fender near the vacuum amplifier (round blue thing with ~5 vacuum hoses attached). If found this setup needed to be disabled upon switching to synthetic ATF - the trans would flare when the car was cold. Just unplugging the electrical connector from the three way valve disables it.

Good luck.

- Jim