Our 14 year old furnace is in trouble. Gas wouldn't flow thru all the burners - so no heat. Burners were really rusty - so removed & cleaned them up. Furnace ran for 2 days & then no heat. Bought new burners & installed them - furnace ran for two days & then no heat again. Igniter is new & works ok. Flame goes to first burner & then dies. Any idea what to do next? Tks.