Since the pressure wouldn't exceed the regulated pressure (60-66 psi) with the return line valved off, it's probably a bad pump or clogged strainer, low voltage to pump or bad ground at the pump.
I would think that they would have checked the voltage (voltage drop) at the pump and also the ground there. There's a test jumper near the driver firewall at the fusible link that will energize the pump when connected to one of the adjacent battery positive posts. Use a fused jumper wire.
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Check ground 404, see if it's clean and tight. I couldn't find a better image, but it's located in the back somewhere...
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Next time it acts up, swap the relay with a new one. If it doesn't help, return the part.
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