My NYC apt is freeeezing. How soon should the landlord fix it before I break my lease?

This past August, I moved into a rent stabilized, ground floor apartment in NYC. Since the heat has come on, it has been a steady 68 degrees, which I am not thrilled about (I like 75-80) but that is the legal minimum. Now that the temps have dropped, my apartment is not a steady 58 degrees and I am DYING. When the temps drop further, I expect the indoor temp to also drop. The landlord has been responsive but the temps aren't changing. The radiator doesn't seem to stay on long enough to increase the temp, only hold it to 58. I have friends upstairs who have toasty apartments. I want to cry. i don't know what to do. How can I deal with this? If it doesn't get fixed, how soon can I break my lease? I can't spend a winter under a blanket wearing gloves, hat and a scarf. :(