The pressure drop was caused by this... Some sort of powder in the bottom of the gas tank. I believe it might be gravel dust. Every time I remove an interior panel on this car or lift the carpet there are copious amounts of gravel dust. Also, the undercoating has been worn away behind the front tires on the floorpan, in certain places even through the paint. This leads me to believe that two owners ago this car frequented a long gravel road...

Luckly I have a low miles FC parts car that was wrecked. I pulled the tank and fuel pump on it and everything inside that tank looks brand new. Rather than try to clean out my tank and replace the pre-filter, I'm just going to swap tanks and pump assemblies and be done with this headache.