2005 Vauxhall Corsa water boiling over

Hi I have vauxhall corsa 1.2sxi twinport 2005, I recently had a mechanic run some tests because it was losing power there was little bit of cream round the oil cap, he did compression test and emissions test which all come back fine, I also explained to him that the car was boiling over not the temp gauge on dash but the water tank itself when driving it sprays water all over engine, so he had look and told me that the spark plugs that I had on it were rubbish so he changed them and put new water cap on, I thought that was fantastic as he saved me 380 as I thought it was head gasket. I been driving car and its lovely but checked under bonnet tonight and noticed that the water was again spraying over engine whilst I was driving it also when hot looks as though there is no water in tank as well but as soon as I open cap the water rises back to the max level any ideas anyone really need to know whats up with my car x x