This actually sounds quite normal, if I read it right. When the engine is cold, the Idle Air Control (IAC) valve automatically allows more air to bypass the throttle plate at idle, resulting in an idle speed of ~1500 r/min. As the engine slowly warms up, the IAC valve gradually reduces the amount of bypassed air, until the engine reaches its operating temperature. At this point, with all loads off (no a/c, cooling fan, or electrical items are on), the idle speed should be at 670 rpm +/- 50 rpm, according to the Helm's (factory) service manual. Is this about where your idle is?