Furnace running with manometer on pressure switch

My furnace has been cutting out before the cycle is finished, and it throws an LED code about insufficient vacuum measured by the pressure switch. What I'm trying to figure out is whether there is a problem with the presure switch, or something else...
Furnace info:
Coleman 8.s series
80% efficient
The pressure switch is rated to trigger at a vacuum reading of 0.5" of water.
Initially, the exhaust fan pulls a strong inch of water, but after the flame kicks on, and the exhaust air heats up, the vacuum drops.
Is this expected?
Should the vacuum get as weak as it does during heating?
How close to 0.5" should the pressure switch activate at?

[@cpspot]

Update - The problem turned out to be a cracked solder joint at the power connector for the exhaust fan motor. It made a good connection when cold, but as the motor housing heated up, it flexed in such a way that made the connection intermittent, to the point that the motor slowed enough to make the exhaust vacuum drop below safe levels. A Center Point Energy HVAC tech replaced the entire exhaust motor / housing assembly and that resolved the issue. I have no way to prove it, but suspect that the power connector's solder joint got broken when we had our vents cleaned in the spring, several months prior to noticing the problem, when the weather got cold. I assume that when the guy was cleaning the furnace, he probably bumped the power connector with his compressed air wand, and broke the solder joint. I hope this info can help someone in a similar situation!!

[@mrstang1977]

Did you pull the pressure switch and test continuity when manually activating the switch? Contactors can act very sporadically when they start going bad.