How about a dirty throttle body throat causing the throttle plate to occasionally bind up before fully closing which prevents the throttle position switch from tripping (you're supposed to hear a "click" as the throttle plate opens and closes) which in turn tells the engine it's supposed to be in idle control mode? That would be my first guess. After that you could be looking at an idle air control valve that is sticking in a few magical spots, such as when the engine is cold. The problem could also be a faulty rpm sensor (damaged cable above sensor on the bell housing), usually more notable when the engine is hot. Other possibilities include an intermittent fuel injection system relay (aka. radio noise suppression relay). In any case, I believe you have the LH 2.4 system with a '91 B230FT so you should check for trouble codes at the OBD diagnostic socket (see the FAQ).
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Dave -not to be confused with a real expert