The vehicle is a very well-maintained 1990 420SEL with 134K miles. I've owned it for less than three weeks, and am just now getting a "feel" for the car. Lately, I have noticed a quirk in the transmission shifting that I hope someone here can help me with.

From a standing stop, the transmission will not shift until I let of the accelerator, then I get two shifts about a second apart. So, as best I can tell, I am shifting from 2nd to 4th gear in about one heartbeat. I have checked the fluid level, and it is full.

Background: There is a vacuum issue with the ACC in that, whichever mode the controls are in, the air -- both heat and a/c -- comes out the wrong vents, and it is randomly switching from one mode to another. This is most assuredly a bad vacuum hose or pod, and I know how to test that, but just have been pressed for spare time of late. The engine also "hunts" at cold idle; tach swings back and forth between about 700 to 1000 or 1100 RPM, and only does some better when warmed up.

I'm leaning toward some vacuum issue being related to all three of these problems, but need advice as to where to start, and it would be helpful if someone could direct me to a vacuum diagram as I don't have the first clue where to begin looking.

Back to the shifting problem: I have never tried seeing at what point the transmission would shift on its own because I have always instinctively let off the gas at just before 3000 RPM. I'm hoping this is going to be a simple fix, but admit I don't know where to start. TIA.

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1980 300SD 146K miles.