equipment not operating properly

I ordered this product in early January 2013, it was for a little over a month worked well (I already had the same before) but five days since the unit begins to warm, and then after 1 or 2 minute stops himself, then returns to the road alone, and it repeatedly without reaching a boil. It's infuriating, and we lose a lot of time to have a hot water that is not boiled. What should I do, there is a good guarantee.
Kind regards