furnace starts but shuts down after 1-3 mins

My pilot light is on and my blower kicks on when I turn the thermostat up, but the burners won't light. Could some one please help before my wife kills me?

I've just recently replace my old gas furnace (intertherm MAC 1148, I think) rated at 78K BTU output with a Miller M3RL-080 rated at 72K output. Both are downflow models, installed in a closet with combustion air being provided by 3" PVC piping. Furnace works great, good firing rate, no leaks in duct work. Old furnace could easily maintain 80 degrees. New furnace cannot keep up. Can only hold at 65 degrees (outside temp 30 degrees). Heat output at registers seems adequate. Problem is furnace is drawing a lot of cold air from the outside. I turned the blower to low fan setting, no help. With old system, there was no evident inflow of cold air from the outside. I recently caulked and plugged all the airleaks I could find. With the old furnace, I was able to have my woodburning stove operating. With new furnace, it draws the air down the chimney into the house. Any suggestions on why the new furnace seems to be drawing a lot more air from the outside than my old one? Also any suggestions to help remedy would be greatly appreciated. Thanks