I don't know why the fuel pressure would change just because it's down to half a tank, but when this problem occurs if you crack the gas cap loose just a little can you hear and pressure or vacuum escape and does the engine run any better after loosening the cap? If so, it sounds like the cap vent is bad, change the cap if this is the case.
Other than that I suggest you stop by an AutoZone or Advance Auto and have the engine trouble codes retrieved from the engine computer module and if it's not obvious and they sell you a part, post them codes back in here and I'll help you troubleshoot the codes.
You can still check the fuel pressure if you have a fuel pressure tester.
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Sunday, January 20th, 2019 AT 6:27 AM