Hi and welcome to the forum.
Have the mechanics been able to replicate the problem themselves? Ideally they could test the fuel pressure.
[It is possible the fuel return valve is full of crud too?]
Has the fuel level always been at a similar point when you've had issues or has it happened when full/half/nearly empty etc?
[Your Forester has more than 1 fuel pump, you have what is called a "Jet pump" in the tank too, which pumps fuel to the part where the main pump is. Imagine if your fuel pump is the shape of a pair of saddle bags on a horse. The main pump only pumps from one side. If that side gets low, the jet pump moves fuel from the other side - a simplified explanation
The fuel tank diagram is below so you can see what I mean]
Has the issue typically occurred when the car is on an incline or a camber (So not perfectly flat)
Have you seen the old fuel pumps that have previously failed or any mention of crud being sucked up and found in the filter "sock" ?
When the issue occurs, have you tried loosening (to break any possible vacuum), then tightening the fuel cap to see if that happens to help ?
[You mentioned full tank left in Texas heat, so perhaps something got over-pressurised or a vent valve is sticking?]
Have the mechanics been able to replicate the problem themselves? Ideally they could test the fuel pressure.
[It is possible the fuel return valve is full of crud too?]
Has the fuel level always been at a similar point when you've had issues or has it happened when full/half/nearly empty etc?
[Your Forester has more than 1 fuel pump, you have what is called a "Jet pump" in the tank too, which pumps fuel to the part where the main pump is. Imagine if your fuel pump is the shape of a pair of saddle bags on a horse. The main pump only pumps from one side. If that side gets low, the jet pump moves fuel from the other side - a simplified explanation
The fuel tank diagram is below so you can see what I mean]
Has the issue typically occurred when the car is on an incline or a camber (So not perfectly flat)
Have you seen the old fuel pumps that have previously failed or any mention of crud being sucked up and found in the filter "sock" ?
When the issue occurs, have you tried loosening (to break any possible vacuum), then tightening the fuel cap to see if that happens to help ?
[You mentioned full tank left in Texas heat, so perhaps something got over-pressurised or a vent valve is sticking?]