Might be hard to separate run time from heat soak, but might be a good test. If you have any braided hose, could even be the inner liner separating and creating a "flap" that impedes flow.
Once had a similar problem and it turned out to be a rag that some joker put in the gas tank; it would get sucked up against the sock.
I believe your car is modified, not an OEM resto, right? In that case, I'd put some fire-sleeve/heat-sleeve on the lines/hoses in the engine compartment. I've had good luck with that stuff, even running it for the whole length of fuel line under a car in some cases.