OK, here is the mystery:

1964 230sl, fully sorted-out except for idle problem. Car starts fine, runs fine, warm or cold, idles at 900 cold, 700 warm (10-20 minutes around town driving) but if I take it up to high temp., i.e. run it at highway speed for 10 minutes or around town for 20 minutes, especially on a hot day, idle goes to 1000-1200. Adjusting the idle back to 750 in hot conditions makes it undriveable (idles at 400) when back to warm. Any guesses? And thanks for your response,

g