1997 Other Mitsubishi Models Loosing Engine Coolant

I have an automatic 2.0L 1997 Mitsubishi Spacewagon that keeps loosing all its coolant once every 1-2 weeks. The leak is coming from the tube out of the overflow tank and only it leaks in the first 2-3 minutes after shutting the motor off, and only if the motor was completely warmed up. The only time it leaks while running is if I'm caught in traffic (stop and go), and it stops leaking once I get going again. I can drive for hours around town or at 50-60 MPH on the highway and not loose a drop.

So far I have replaced the rad cap, removed the thermostat and flushed the rad a couple times, all to no avail.

Any help in this problem would be greatly appreciated.

Thanks,
Dan