Pressure switch failing. Ventor fan slowing down. Need a manometer to check the draft thru the furnace and rating of the switch. May have an inlet screen where the pipe attaches to the furnace/burner box with debris in it slowing down the draft. Some Carriers and Lennox do.
"Cut it twice and it is still too short".
Could also be recirculating the exhaust into the intake. Have seen that happen more then a few times due to someone cutting off the vents or placing something in front of them to block the view of the pipes.