4-speed A/T shift trouble

[colin4coolin]

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...?
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.
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.