Put me down as another zetec owner with the exact same behavior you describe. Idle in mine actually improves with the A/C on, too... guess thats cuz it kicks the RPM's up slightly.

96 Contour GL 2.0L MTX with Early SE 7 spoke wheels, SE sideskirts, Koni Sport Kit 70 Mustang Sportsroof with mild 351W and '01 Mustang GT wheels