Vintage Cars: 1986 420 SEL, vacuum leak, center vents

Question
hi ron my problem is that when i drive the car it feels as if the car is still going as if the cruise control kicked on..on a flat surface i can drive at 40mph and let my foot off the gas and it still goes like 35 then 30 but seems to be as if my foot is on the gas pedal..when i drive down hill it feels like if i were applying the gas and brake alternately not smooth as it should be. I have changed fuel distributor with a used one and is better than my original..could there be a vacuum leak and had a mechanic try and replace the vacuum hose that leads to my trans.. idon't think he used maybe appropriate line is this line just plastic hose or should it be a metal hose. my ac blows only to my defroster vents instead of the main center vents any recommendations on what to look for.

Answer
Is your linkage to the throtel body free, I have seen them bind at the accelerator linkage as well
Make sure all that moves freely and that your throttle plate moves free, push it down, it should float back up.( quickly)
A vacuum could case this but unlikely unless it is a major leak ( manifold supply line) or lines off distributor.  The hoses need to be able to withstand vacuum pressure, most plastic lines cannot.
Check your owners manual , center vents do not "always" blow cold AC ,the side vents should.  If you are getting just to the defrost , most likely a vacuum pod has gone bad, not an easy fix.  There are some DIY instructions on the web, google vacuum pod for ac vents - Mercedes