A couple things come to mind:
Are you frequently tipping your Vivi Nova to keep the string wicks wet? It sounds like the "metalic taste" you're experiencing could be from dry hits from the silica wicks being too dry.
What juice flavor are you using? Some flavors react destructively to the polycarbonate plastic the Vivi Nova tanks are made from and can cause chemicals in the plastic to leach into the juice. Do you see any frosting of the tank, or any spider cracks? Has the juice turned cloudy?
At what voltage are you using on that 1.8 ohm atomizer? Typically, from my experience, I've had to lower the typical voltage on a clearomizer when compared to a similar ohmed cartomizer. Also, any voltages above 4.0 volts might be too high for that resistance.
* FYI, your thread title is misleading. You switched from cartomizer in the title to clearomizer in the question.