Just an FYI to those of you having problems. I went through the same basic checks that many of you did...pressure switch, paper clip, check the flu lines to see if they were plugged, etc, etc.
When you've tried all the basic stuff (I even replaced my pressure switch with a new one (same model for $40 online) and it doesn't fix your issue, it's probably a cracked heat exchanger. Mine was cracked for over 2 years while I had repair guys out trying to fix it. In the end, the 20 year warranty covered the exchanger, but not the labor. I feel fortunate to have that done by a great service guy for $350. The problem NEVER returned after the heat exchanger was replaced.
The crack was right on the header of the heat exchanger...about 18 inches long and up to 1/4" wide. Note that the header is plastic. Go figure.