This is a toughy, and I hope I'm wrong, but my symptoms, the same as yours, were due to a crack in the head between the valve seats. The engine compression goes into the water jacket and pumps the coolant into the expansion tank, then into the overflow tank which then overflows.
 I imagine a head gasket (or whatever they are called on our wbx's) failure could do the same thing.
 
johnnyjewel
86 westy
05 jetta
 

> Date: Mon, 22 Aug 2011 15:08:41 -0300
> From: 86westy@GEOEXPLOITS.ORG
> Subject: Coolant Strangeness
> To: vanagon@GERRY.VANAGON.COM
> 
> This is likely an FAQ...I'm on the road and searching is awkward with
> cell phone.
> It seems when I drive for an hour or so at highway speed, and stop
> abruptly (to get gas for example) coolant rushes from the left tank
> into the overflow one on the right, sometimes overflowing that one
> onto the ground. In one instance the dash light began to flicker and
> when I pulled over, the right hand tank was full, and the left one was
> down 1/2 litre or so. It's not overheating, but temperature rises at
> idle after a highway run.
> 2.1 litre, recent (last 6 months) new rad, water pump. There are no
> evident leaks, and oil on dipstick looks clean.
> Any suggestions on what to check?
> The needle is generally parked right at the center of the LED...
> Thanks
> Tom