Lets see heat up at idle and cools down under drive. You mean it over heats at idle and cools to normal operating temp while driving. or it heats up to normal temp at idle and over cools at driving speeds?
Given the first (over heats at idle):
The high low fan resistor may be broken and is only doing low speed and is not enough to cool down with out the extra air flow.
If not resistor the fans may be on there last legs and just need new electric fans.
Could just need a coolant flush and new thermostat. Do you know if they ever used any fix leak or stop leak type stuff?
possibly bad sensor, coudl use thermal camera or thermalcouple and verify temp or just replace temp sensor.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)