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Those symptoms point to a partially clogged intake air tube, or the vent pipe is too long or with too many elbows in it (possible if its a new system only)... tell me about how old the system is.
Or a bad pressure switch.
Please attach a well focused, close up photo of the wiring diagram glued inside the service panel, the picture needs to be taken from straight on, in very good light.
You can use the paper clip- icon at the right end of the tool bar at the top of this dialog box to attach it here. ,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.
We can go from there.