what would cause the idle to just start racing for no reason? i will be stopped at a light and idle will jump from 600 or so to about 1800. gets kind of scary. sometimes if you restart the car it will go back down to normal and sometimes it wont. you can drive to the next light and it will be normal again. very unpredictable. seems to be no pattern. seems to do this about 25% of time. hot or cold.cleaned throttle plate, no effect. nothing else seems out of place. help?

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85 300 sd 168k current driver
85 300 sd parts
81 380 sl 125k
84 190 d 160k just bought!