Now you know why I said this was important information. If your mechanic had done this early on (and based on the symptom, he should have), you would have saved a LOT of checking, guessing, testing, asking, replacing, getting stuck, walking home.

Pressure regulator is out of the picture, this is a delivery fault. My instinct has me at the same point as I started - main fuel pump related. Being so consistent puts a few other items a lottle lower on the list. However, if I was working on this car, I'd monitor the voltage at the pump terminals to be sure that the voltage isn't dropping due to heat and load showing a wiring weakness. You can do this by taking two wires, stick a bared end into the push-on connector and replace onto each electrical post on the pump. Put a voltmeter on the other ends to monitor pump voltage. If the pump voltage remains over 11V and pressure drops, pump itself is bad. Because of the consistency, I'd say that one winding of the armature is grounding when heated and causing a significant drop in motor efficiency. It's one of the few explanations that logically explains your symptom without ghouls, goblins, ghosts or maybes.

Relay, athough possible, usually fails because of bad solder connections inside and cause unpredictable LOSS of fuel pump, not low pressure.

Intank pump can be checked separately but high fuel tank level almost always covers its failure. You can disconnect the big hose to the main pump, put a bucket under the hose, crank the starter for 15 seconds. If you have a cup or more of gas in the bucket - it's good. Faster test; if the car runs the same with a full tank, this isnt the cause - even if the pump is bad.

Try one other thing before going further - on the itsy-bitsy chance that the tank vent is blocked, loosen the fuel cap right after the pressure drops and recheck it.

So you don't have to ask what some of this means; if voltage and fuel cap checks don't reveal anything, then the main pump is your answer.