Why is the furnace blower continously running?

This is a two-stage gas furnace I acquired three years ago, when I moved into the house. The furnace was new at the time. As I live in a city along the 53rd parallel latitude, the furnace is often in use especially during the colder months. The furnace would turn on or off by itself according to the temperature specified by the thermostat. It bears mention that the original thermostat was replaced for a digital one, about one and a half years ago. I am not completely sure when the problem arose with the furnace blower, but it came into my notice six months ago. As it stands, I am continually forced to manually turn on and off the furnace, which leads to temperature fluctuations in my house. I have searched extensively online as well as through the manual for the furnace and have been unable to find a solution, so it would be very appreciated if someone would shed some light on the issue.