pressure swithes have a rubber diaphram inside them. eventually they will stretch enough that they don't make the circuit. They can also become erratic when there are minor obstructions in the orifice at the furnace,inside the tube etc.If the inducer doesn't get up to speed, the pressure switch also will become erratic. I've seen numerous ones changed, only to find out it had a different problem. You need to verify the failure mechanically and electrically.