1) possibly a generic gas cap that is creating vaccum. Try a replacement OEM cap (these usually should be replaced every 45-50k anyway due to erosion and lack of seal forming)
2) fuel pump has worn itself out due to having to work harder with no tank pressure from failed cap
3) bad MAP (? spitballin here)
The key for me is it will start with pedal depressed, and won't trigger past 3500. That triggers a few red flags in an unusual combination.
It is possible that the combination of poor fuel pressure reg caused the MAF to become off calibration in relation to ECU. You could (just to rule out a few factors) reset the ECU in the 3 step process of resetting ECU op, idle, and throttle setpoints
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