98 Achieva no heat

I just had my heater core replaced last winter and the heat worked fine then, but now my heat seems to not be working at all. It used to take long to heat up but now after running my car for 35 min it was still blowing cold air. My temp guage also seems to rise to almost the red line then fall back to the middle line. As well as this my fan shuts off when I try to put it on full power(it works on levels 1,2,& 3). Any advice on what the problem could be?