4-speed A/T shift trouble
 

I know that the shift-points for my 1984 300D, are regulated by vacuum, and there's some fittings with orifices somewhere, that get clogged with soot, maybe causing my shifting problem...?:confused:
Oh yeah--- the symptom is this:
She doesn't like to be in 3rd gear, so when that's where she needs to be, she sticks in 2nd, or if i let her coast a bit, gets into 4th. Either way, it's quite embarrassing when I floor it and eek out 30 MPH in a 55 zone.:eek:
So where's the part that's clogged, and what's the best way to clean it ?
Thank you, people,;)very much- for your suggestions, and expertise.:)
If this has been addressed before, please direct me to the link. I have trouble with the search function.

Vacuum controls the quality of the shift, the bowden cable controls the shift points. Make sure your bowden cable is moving freely and the kickdown switch is not sticking on.