i drive a 96 ford taurus and it does about the same thing my idle is at about 700 and it will sometimes at stop lights just drop to like 200 and bounce up to 400 and back to 200 it does that for a few seconds then goes back to normal. my friend that works on cars alot said that the idle air control(IAC) was the cause and that fords were well known for the (IAC) to malfunction. i took it off one time and cleaned it with breke cleaner and it was ok for a while but it still does it from time to time. unless it happens all the time i wouldnt worry too much, i would clean that iac and if it still does it and dont get any other helpful info and are concerned go ahead and take it to a mechanic or someone. note- the IAC should be close to the throttle body and intake manifold.