Bucktail has a good point. I did upgrade the blower fan. My heating guy said for the size house I have I only needed a 1/4 horse fan...I insisted on a 1/3 horse so I'd get flow to the upstairs. Make sure you check the ducting after they install it too. My wife noticed TONS of drafts coming out, so we duct taped it all up and it made a huge difference. Good luck!
Oh yeah...Gerritt, my furnace stays on the WHOLE time I have a fire going in my fireplace. Other than that, it operates normally. We have been told that because our fireplace is in the basement, the air flow comes from upstairs. The colder air from the higher levels travels by our thermostat and tricks it into thinking it's colder than it really is in the house. Have you ever noticed this happen when you have a fire going? May be worth looking in to.