Vintage Cars: high idle, mass flow sensor, air control valve

Question
My son just drove back home from school in Canada to Durango Colorado a 24 hour drive, in his 1988 six cylinder mustang lx that he purchased there. The problem is it doesn't idle right it starts off at 2000 then will drop down to 1400 and some times to 1200 then you have to hard shift it into gear.I tried to adjust the idle screw on the throttle cable but it was already adjusted down. Is there a idle speed connection to the transmission. Theres a cable wire connected alongside the throttle cable do you know what this controls.Im not sure what to look at next and would really appreciate any information you might have.

Answer
Marlo,
first off do you know what the cold and hot idle are suppose to be?  2000 seems high for cold, 1200 too high for hot?  That would be good information,  anyway things to check

Throttle cable and linkage at throttle body make sure it is not binding or caught up. Correct if needed
Check vaccum lines (very important) to make sure they are secured  the hose that runs from the back of the block to the under side of the intake is one that gives problems from time to time.
I think yours has a  IAC (Idle Air Control) valve and may be  sticking. Look for it  on the throttlebody , has a 2 wire plug and is held on with 2 8mm headed bolts.
You may need to replace.
I will need to do a bit more research to see if your has a mass flow sensor, they sometiems can give these symptoms as can faulty O2 sensors.  Does it have a "code plug"  If so take it by one of the chain auto parts stores and see if it is throwing a code.  Let me know which one
Of couse there is the multitude of other things such as fuel filter, fuel pump, compression issues, ect ect, do you have any service history on the car? ... and you failed to tell me the mileage
Good luck and Happy Motoring