The combustion air intake is restricted, too long, or sloped so that water has collected in it... OR... the pressure switch is going bad, has a bit of water in its 1/4" rubber hoses, or needs the nipples the hoses are attached to cleaned out with a paper clip wire.
Please attach a well focused, close up photo of the wiring diagram glued inside the service panel,you can use the paper clipicon at the right end of the toolbar at the top of this dialog box,Include a picture of the thermostat wire connections to the printed circuit board in the same area.
If there is no paper clip icon, look for the '+attach files' link at left of your send button.
I can mark it up with trouble shooting instructions. You will need a multi meter ($30 at a hardware store) and some experience working with appliances, let me know a lot about your skill sets in those regards.
We can go from there, if I miss you tonight I will pick your response up first thing in the morning.