Check your coolant hoses and clamps!, i was at an event last month and minutes prior getting on grid i noticed one of my clamps (OEM, factory) was not holding the coolant pressure.
Bring a small tool box to the track, our mk1s are not that new and you could need a screwdriver/wrench for a quick fix.
You say your fluid is clear, and you are going to de1, i would just go like that. Mine was black and took the car to a couple of events, in fact the fluid is still there.
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