Sounds like water in the chamber. How long have you run it for, if it was condensation, after a 20 mile drive you would expect it all to boil off.
I would take it for a fair distance drive, if it clears up then it was moister, if not sounds like HG trouble.
Wip the head off, get it skimmed and then put a new HG on
![Cheesy :D](https://www.golfgtiforum.co.uk/Smileys/modern/cheesy.gif.pagespeed.ce.I9DXYfcQe-.gif)